Going through campagn idea, and one of the big bads will be the following: 

War Master Tibbolt of House Spurrose (AKA the Butcher)

Warlord with levels in Fighter and Battle Master.

Backstory: The product of a noble woman and the Black Dragon that calls swamp in the territory home, it was hoped this would solidify an alliance. It did not, but the Undead War had occurred and the Dragon was forced to keep its end of the Bargan. The boy Born, called Tibolt, was trained in the arts of war and he excelled at it where he became a legend in the field of battle, rising up the ranks from squire to knight, to young lancer to general to Master of All Arms of the City within a hundred years, and he desires more.

He earned the moniker “The Butcher” in one battle where he basically jumped from level 10 to level 20 and on a pile of undead he had slain over and over again till even the most mindless of zombies refused to head their lich’s call. Brilliant, cunning, and brutal, while he will never sacrifice units needlessly he will sacrifice any piece on the board, be they pawn or queen, if it secures him victory. He also desires to rebuild the Crime City in his image. Currently a humble advisor and Lord of Warlords (and a dozen other titles I might call him). Currently working with a cabal of individuals that wish to see the cities destruction, lead by a Cambrian trying to get the whole city into the Hells. He's hoping to eventurally betray them so he can rebuild the city in his image. 

Outside of battle, he is charming, an excellent painter, and even a bit of a flirt. However he is not afraid to get his hands dirty, and there will be parts where the party may see him in action (at least one arena fight, and one where he may ask them to help him train).

Plan: while I will wait to see what the new Bestiary has, by and large I want a warlord with a great sword and throwing axes, late armor, and able to handgle a group of 3 to 6 by himself as “an exercise.” I might even have a “friendly sparring” practice for the party to see how he handles a fight, and what they need to defeat him. However when they do face him he will have his honor guard.

For now thinking new Dragonborn build on old warlord, but may keep the Half Dragon. For fun and to show his prowess, I wil take inspiration of a level 20 that got another 10 epic levels atte aster Fghter, and tr y to tweek some of these abilities for better use in combat.

On the top of my head:

Warlord

Base Build

Medium Back Dragonborn, Lawful Evil.

HP 229 (27D8+108)

AC 18 (plate armor)

Speed 30, Climb 30

Strength 22 (+6) Dex 18 (+4) Con 20 (+5) Int 17 (+2) Wis 16 (+3) Charisma 20 (+5)

Saving Throw Str +12, Dex +8, Con +8

Skills Athletics +9 intimidation +11 Perception +9 Persuasion +11

Passive Perception 15

Speaks Elvin, Draconic, Common, Infernal

Resistance: Acid, Necrotic, Fire

Proficient in Painter’s Tools

Challenge 19 which is 22,000XP  Proficiency Bonus +6

Indomitable (3/3day) Can reroll a failed saving throw, must use new save

Survivor: Regains 10 HP at the start of its turn if it as fewer than half its HP but at least 1HP

Weapon Mastery: 6 Great Sword, Long Sword, Hand Ax, Short Bow, Great Crossbow, Pistol

Multi Attack: 4 Great sword or short bow attacks

Greatsword: +9 to hit. 5ft reach. Deals 12(D6+5) Slashing damage.

Great Weapon Fighting Stype: Using tw handed and the versatile option of weapons with both hands allow you treat wounding rolls of 1 and 2 as a 3.

Short Bow: +7 to hit. Range 80/320ft. Deals 6(1d6+3) Piercing damage

Wyrmling Breath attack: Acid Breath (recharge  on a 5-6): 15 ft line that is a 5ft wide Each in that line makes a DC 19 Dex Save (takes 22(5D8) acid damage. Will ump this up later.

Bonus Action: Second Wind: 4/4 Regain D10+20 HP. Can instead use the D10 to try and succeed an ability check. Or Tactical Shift allows you to move up half your movement without evoking opportunity attacks.

Action Surge: 2/2 Get a second Round of Action. Can only use one of these per turn.

Indomitable: 3/3 Fail a save, get to use this for a reroll. Keep the second roll n matter what.

Studied Attack: If you miss your next attack role against the target has advantage until the end of your next turn.

Epici Boon of energy Resistance

Know Your Enemy: Bonus Action know the vulnerabilities, Resistances, and immunities of a creature within 30ft of you. You can do this once per long rest, but bur a superiority die and get to do it again sooner.

Athlete: Hold Up: If Prone can use 5ft of movement to stand back up.

Jumping: Can make a running long or high jump at only 5 ft

Charger: Dash increases movement by 10ft, and if you move at least 10feet  in a strait line to your foe you can either gain 1D8 to the damage delt or if they are no more than 1 size bigger than you push them back 10 ft. Pick one.

Epic Boon of Dimentional Travel:

Manuvers

Once per turn when you use a maneuver you can roll a D8 instead of one of your D12’s  and add it to the move instead.

One manuver per attack.

6/6 Superiority Dice D12

1 Ambush: Wen you make a Dex Stealth Check or an initiative you can expend a superiority die and use its roll. Can’t use if incapacitated.

2Commander’s Strike: When you attack on your turn, you can replace one of your attacks to direct one of our companions to strke. When you do so choose a willing creature who can see or hear you and expand one superiority die. That creature can immediately use a reaction to make an attack action. Add the superiority die to the damage rolled

1. Commanding Presence: when you make a charisma (Intimidate, performance, persuasion) check, expend a die and add it to the roll made.

4 Feinting Attack: As a bonus action target an enemy and expend a superiority die. The enemy must be within 5ft of you. You have advantage on your next attack against them this turn, and if it hits you add the superiority die to the damage.

5 Lunging Attack: Use a superiority die to make a dash action as a bonus action. Move at least 5 ft in a strait line before hitting an enemy with a melee attack as part of the attack action you can then add the superiority die to the damage delt.

6Sweeping Attack: Hit a creature with a melee attack, use up a Superiority die an and attempt to damage another. Target must be within 5ft of the initial target, and within your reach.  If the initial roll would hit the second creature, roll the superiority die and that will be the damage it takes. Sam damage type taken by initial target.

7 Tactical Assessment: When you make a Intelligence (History and Investigation) check or a wisdom (insight) check, use one of the superiority die and add it to the check.

8 Ripost: You can use a reaction to hit an enemy back that misses you in melee. Use up a superiority die and roll a weapon attack against that creature.  If you hit, add the superiority die to the damage delt.

9 Trip attack: Hit a creature and add the superiority die to the damage to it. If its large or smaller make it make a Strength save or it falls prone. Save is on 8+6+6=20

Legendary Actions

Comand Ally: Target one ally it can see within 30 ft of it, and if they can see or hear the warlord the target can make a weapon attack as a reaction and gains advantage on the attack roll.

Fighting Foe: Coasts 2 Legendary actions, Warlord Targets one creature it can see within 30ft. If they target can sea or hear it the target must succeed on a DC 16 wisdom save or be frightened of the warlord until the end of the warlord’s next turn.

As I am aware PC builds are not meant to be made as NPC's, this is a starting point and would like to make something that could hold its own in a fight against 3 to seven players and give them a fun challenge. Will add veterans (also half dragons) as his personal guard, maybe 10 of them. Maybe a handful less. 

Thoughts and ideas to make this work.

Its a fine start, but like you said, PC builds as "monsters" does create issues, especially at higher CRs where things a bit un-balanced anyway even if you follow the guidelines in the (2014) DMG.

At what level are you expecting the PCs to be when they encounter your Big Bad?

I think "PCs-as-Monsters" tend to have less hit points but potentially deal more damage than an average monster so you have to adjust for that. For example, you have assigned your Big Bad as CR 19 with 229. With a low initiative, they may get killed before they have even taken a turn against a party of 4 to 5 CR 17-19 characters!

Some PC abilities also don't translate that well. For example, your Big Bad has Indomitable, but if they are intended to be Legendary you may as well give them Legendary Resistances which work much better.

Also, even a Big Bad ain't going to be around that long, so overloading them with dozens of abilities tends to be pointless because they will never use them all (and its just more for you to have to think about when playing them). Best to focus on just a few abilities that you think they would actually use (3 or 4 actions and a couple of bonus actions or reactions).

why dimensional travel?

My rule of thumb when creating is 2-3 attacks (1 melee, 1 range, 1aoe) 4-5 spells, 2-3 legendry's and the hardest hitting legendary refreshes on a 6. If players see me rolling a 6 side they know exactly what its for. Hp at least equal to the total party - maybe +10 saves all around or advantage on saves +5. Maybe 2-3 legendary choose to pass on fail saves as well, or in my higher level games out right immunity to banish - fear - petrify etc. Basically no 1 cast I win buttons. 

It all depends if its a boss or a mini boss or of course for a random encounter none of this applies. 

THe RP of a backstory is fine but as others have said if you overcomplicate the creature you likely wont use all the abilities and combat will take to long. 

Let's simplify it down:

Warlord

Medium Back Dragonborn, Lawful Evil.

HP 285 (30D8+150) [Increase b/c I took away some of the regen]
AC 18 (plate armor)
Speed 30, Climb 30

Strength 22 (+6) Dex 18 (+4) Con 20 (+5) Int 17 (+2) Wis 16 (+3) Charisma 20 (+5)

Saving Throw Str +12, Dex +8, Con +8
Skills Athletics +12 intimidation +11 Perception +9 Persuasion +11, Painter's Tools
Passive Perception 19
Speaks Elven, Draconic, Common, Infernal

Resistance: Acid, Necrotic, Fire
Challenge 19 which is 22,000XP  Proficiency Bonus +6

Legendary Resistance (5/5 day) If fail a saving throw can choose to succeed instead [This is necessary b/c his INT, WIS, and CHA saves are awful so he could easily be taken out with Banishment, Hold Person, or similar spells]

Martial Expert: The warlord always has advantage on his attacks.

Actions

Multiattack: 4 Greatsword or Shortbow attacks, the warlord can replace one of these attacks with its Acid Breath.

Greatsword: +12 to hit. 10ft reach. Deals 19(4d6+5) Slashing damage, deals 5 damage on a miss. [Damaged increased as I will take away all the extra features to stack damage onto these attacks]
Short Bow: +10 to hit. Range 80/320ft. Deals 14 (3d6+3) Piercing damage. [Damaged increased as I will take away all the extra features to stack damage onto these attacks]
Acid Breath: 30 ft line that is a 5ft wide Each in that line makes a DC 19 Dex Save vs  22(5D8) acid damage, half on success.

Bonus Action:
Teleport : the warlord teleports up to 30 ft to a location they can see.
Regenerate : regain 20 hit points.
Attack : make one Greatsword attack.

Reactions (3/round):
Riposte: If an enemy misses the Warlord he can use his reaction to make one Greatsword attack against them.
Command Ally: When the Warlord's ally is targeted by and attack or spell, the Warlord can command them to Move or Strike, Move = they use their reaction to move up to their movement speed potentially avoiding the attack/spell, Strike = they use their reaction to make one weapon attack at advantage
Frighten Foe: When a creature moves within 30 ft of the Warlord, he attempts to frighten them, the target must succeed on a DC 16 Wisdom save or be frightened of the warlord until the end of the warlord’s next turn.

I think that covers the core mechanics and makes it much easier to use.

Thank you. THe dimentional step was a kind of a idea for a flash step to attack or show up behind someone. Might go instead with the true sight instead.

I'll try working on these today and see what I can do to make him interesting.

I am expecting between Level 15-20 for the group of 3-7. 

I came across this website.  You might find it useful:

Understanding How to Use Legendary Actions in D&D 5e (beforeyouroll.com)

My advice with monster stat blocks is  focus on what they will actually use in combat keeping in mind the typical combat is 3 rounds and keep the stat block minimal. 

• Don't have resource pools unless you intend them to be something players can exhaust. As players primary interaction with the stat block is to attack them  that means you typically only use resource pools for defenses but it can also be worth giving a powerful ability a single use to give value to things like counter spells, dispels or concentration breaks. 
• Don't include out of combat abilities. NPC to NPC interactions don't require rolls only player to NPC interactions do. For most of that you can simply set a DC for the players to interact with based on a general sense of how good their plan is or how good the NPC is meant to be at a thing. Stuff like painters tools don't belong in the combat statblock they belong in the description of his bed room or back story.
• Keep the dice rolls minimal.   The more attacks and other roles you do the more math you need to do and the more time it takes. You are generally better off doing 1 attack at double strength that two attacks for example. With action surge this stat block is doing up to 8 attacks which means your turn will take 8x longer than say a caster who casts a cantrip.  With two of these you will be doing this 2 out of the average 3 rounds that an encounter lasts so it's a less a big moment and more his normal attack pattern especially when you include that he has a breath weapon to take up the other turn.  I'd say at most you want to action surge once but you don't need to at all because the breath weapon does the same super attack function.   
• Players don't really have permanent resistances and immunities. It's also a pain to give a stat block a variety of types of attacks to take advantage of that info. My advice is just say the damage ignores resistances an immunities and  describe him as having a unique fighting style that enables that. 
• Just generally too many abilities. Either combine them or choose which ones you want to use.  4 bonus actions and probably only 3 turns. One is almost certain not to be used and realistically with how attractive that healing is, it will be the only one used.