Beginner gm seeking advice before my sanity goes out the window

Overly dramatic title aside, here is the situation.

First off a little kind of disclaimer. So far everyone has fun but at this point I realized, that most encounter, even bosses don´t impose any real threat to the party and I think it´s mostly my fault, so I try to find a way to ballance things out a bit.

I am not trying to tpk them or kill single charakters on pupose. I just want a bit more challenge. 

Currently running the crooked moon as my first campaing. because of this, I decided to stay as close as possible to the books descriptions. 

The Party consists of six players. clearly my first mistake (but sessions take place when at least 4 are available due to the Campaing being intended for a party of four and it shows in regards of how encounters play out)

Characters in the party are: bard, druid, barbarian, cleric, paladin, rogue. no multiclasses so far.

We are halfway throug the story so far and it get´s hard to even put a scratch on them

well... this party has a quite strong synergy (also the Campaign throws magic items left and right on them). Experiment barbarian uses his maddening fumes trait to force anemies to do a wis save or have disadvantage on every target except him. Twillight cleric has a AC of 24 due to a plate armor he stole (whoever thinks to do the campaing, looking back i strongly advise to change the part with the three knights in the crimson monastery stables to the knights wearing their armor, so you can claim that the armor got destroyed in battle...), the druids´s sanctum of the sheperd, the shield of Marius Renathyr (that grants him a whooping +4 AC) and the druids wicker effigy granting +1. also whenever the unlikely event happens that he get´s hit, he uses the shields reaction to just deny the hit (most common enemies have not that good + to hit.. so they barely ever hit him aside from crits oder close hits) he also has twillight sanctury active in most encounters, so the largest part of damage dealt to any partymember get´s blocked or undone by regained temp HP. 

difference shows whenever this player is not present in a session

Now I guess most people would advise to use abilities with saves, bringing me to the next part of the synergy. The party allways crowding around the paladin, grants them +5 on their saves. prime target for a fireball (or similar aoe), right? 

yeah well, none of the casters so far has fireball.. or dispell magic or counterspell. All the aoe stuff aside from insect swarm on a higher level in a location that would be highly disadvantageus or the caster, is recharge 5-6. Also damage get´s undone by either twillight sanctuary or the tons of healing at their disposal. simply throwing more minons at them? yeah, hypnotic pattern or spirit guardians... it´s kinda trivializing the encounters and everyything I try to ballance things out seem to only lengthen encounters.

Also whenever there is a situation where during travel or that´s not happening on a battlemap and any kind of saving throws is needed, the paladin claims to keep everyone in 10 ft. reach around him. (absolutely trivialized the Hartsblight forest chapert so far, where a dc 10 Con save is needed whenerver the party travels to the next location or they gain a stacking debuff) 

so that brings me to the question, what would be your advise on how to ballance things a bit towards tha party size and situation? 

Giving more HP to everything just makes encounters go longer so far. 

More enemies seems not to be very effective so far. Threw a improvised encounter at them that was labeled as deadly by a en****er calculator. Not a single scratch on them... aside from the druid getting dragged off and mauled by a bat hound. 

my goal is to spice up the challende a bit, giving them at least a greater feeling of being in danger 

My current idea would be to give caster enemies more spells like counterspell, dispell magic or heat metal. lowering recharge time? Turing some abilities into bonus actions?

What would be your advice?

Use terrain to split them up and give monsters some AoE and countermagic. A maze-like area forcing players into single file and then ranged attackers pummeling them from above. Make the maps difficult for them. Battles can be much more than just stats with the right terrain. Play enemies smart. Pounding them with AoE will teach them to spread out, which then allows you to engage them one on one, or a few on one, and makes the battles more interesting. Figure out how to turn their tactics against them.

ok, so with your input in mind and after some research I did and some talks I had, I´ve come to the following conclusions.

Because Encoutners in this campaign are pre designed I don´t have much room, to edit them. also maps are fixed for these encoutners. But I regularly add more enemies etc. 

Whole parties Ac goes down by 1, because apparently the druid doesn´t know his spells (as a beginner I heavily rely on the players explaining their spells or sending them in vtt... a thing I won´t do aganin in the future) and lacks the component, he needs to cast "sanctum of the shepert" 

less long rests, more pressure.

I´ll stray from the "immersive" combat towards more of a optimised one. with the argumentation, that Creatures know what they are good at and also know to identify their preffered targets. 

talked to the cleric about the shileds reaction and that despite the wording, my interpretation is pretty clear about him not being able to use it on himself

also realized, that due to their fabled heirlooms, their save dc´s are pretty high for their level, why I´ll adjust enemy save dc´s. also because the paladin clumping issue. 

probably will adjust enemy caster spell lists ab bit (some are in desperate need of dispel magic and similar) 

I´ll try it like that and see how this weeks session plays out

A given CR should be a deadly challenge for one character of the same level, (almost) always. Also, the more actions a team can take in one round, the greater the advantage. You can take that as a base and do other things, like splitting the party, adding environmental hazards etc.

Also, if a character wants to don armor they've looted, as a DM you can just say that it doesn't fit and needs to be adjusted.

Quote from alvarojosemarquez >>

A given CR should be a deadly challenge for one character of the same level, (almost) always. Also, the more actions a team can take in one round, the greater the advantage. You can take that as a base and do other things, like splitting the party, adding environmental hazards etc.

Also, if a character wants to don armor they've looted, as a DM you can just say that it doesn't fit and needs to be adjusted.

 I tend to add more enemies due to larger party size than the campaign is made for. 
at least every boss encounter has special conditions that trigger at ini count 20. but due to larger party sitze and aura of protection clumping, they rarely have any larger impact aside from respawning minions. 

I did exactly that. The problem: they looted exactly three full plate armors at that part (because of the three knights storing their equippment close by) and I made the mistake to allow to sell two of them. Giving them the money for the allready lower than buy adjustment price (I made it pricey and time consuming). 

"You absolutely need to give yourself permission to change anything in any campaign book you read to work for your party. A campaign boom is written for a generic 4 person party. You have to adjust it to your party size and power. Dont treat the campaign book as some sacred text. It is merely a suggestion of a starting point for you, the dm"

I started to increase the ammount of Monsters they go up against. But it seems like I have to adjust their overall stats? most enemies don´t land a hit. my rolls are too bad it seems. 

"First of all, i think the heirlooms start at low rarity and get more rare in their status as the players level up? Maybe i misread that. No way would i hand out a very rare item to anyone below level 10."

I thought that they would feel the most rewarding.. but ballacing wise these things are a absolute nightmare. players save DC increase to around 19 below level 10. to counter this I started to increase the Creatures saving throws. Or I just let them succeed if a fail would be utterly anticlimatic. 

also I have to mention here, that the feats you get with crooked moon kinda give a lot of heroic inspiration. Fate gambler and tireless reveler being pretty powerful in combination. 

"Also, sounds like youve got quantum player superpositioning. If you say the enemy casts chromatic orb and rhe orb bounced to a nearby party member, the players will say they are spaced far apart. If you say the enemy cast fireball and everyone needs to make a saving throw, everyone will suddenly be standing next to the paladin for his aura.

If you arent doing maps, at the very least get a grid and force players to show their marching order before combat atarts. This will avoid quantum positioning. Force them to declare their marching order at all times. Even if nothing else is on the map. When the encounter does happen, hit them with the marching order they last reportrd

Put down a blank grid and always have their minis doen there showing their marching order."
I started asking early on, how they position etc.  but the grid one is a good advice. thanks, I will add that in the future.

" They should not have a bag of holding at level 1 "

they found one on lvl 3... A cursed one but an overall better one. also the college of whistles bard can create a handy haversack at level 3. 

"Last but not least, i do not allow paladins to use pact of the blade. I dont know what your paladin player looks like. If he is attacking with strength, his charisma cant be much higher than a 14. Thats a +2 to his aura."

well.. maaaaybe  I made the Mistake to use rolled stats instead of point buy or standard array. also they are currently level 9. 
He priorized his charisma score and strenght second. atm his charisma is 20 and his Strenght 18. luckily for me the paladin is the least problem, since he is the one who is the most understanding if something gets too strong. also he would never go pact of the blade, because this player hates everything he deems metagaming or minmaxing.

the barbarian seems to become more of a problem, because he´s trying to optimise his character as much as possible. But I´ll take advantage of him using reckless attack all the time. especialy if i get some creatures with a way higher damage output at my disposal. 

so yesterday was the next session mostly concisting of the fight against the beast of blight. and it went quite well.

so first of all I raised it´s ammount of legendary actions and resistances. also it´s hp got a good buff. 

Next I raised the ammount of druids present at the start of combat, because they are super weak aside from their recharge ability (also increased the save dc of that ability, spreading a bit of poison and damage without having to go against AC). but the main use of them was tha fact, that the beast of blight has a damage threshhold equal to five thimes the ammount of present druids. never saw this party more confused, than when I told them, the sinners 22 force damage showed no effect on the beast. 

then I combined the beasts legendary and bonus actions to first grapple the Paladin, teleport out of their range and restrain him. also later in the battle I did the same with the cleric. 

also the heartyblight condition finally started stacking up and significantly reduced their max hp

in the end they emerged victorius but badly beaten up. exactly the outcome I wanted. Party was happy, I was happy. apparently their favourite boss fight so far. 

so It seems things get finally better. 

Any advice on how much I should raise save dc or saving throws of creatures?

Ok, so I should be generous with the dc increase. last session I decided to raise a save dc of one of the low tier monsters to 15 instead of 10. because, well... at least they should have a chance of getting full damage. 

"Wow. He rolled that in front of you?"
Yes. wo use vtt, so he showed me doing his rolls on DnD beyond  via stream. he got pretty lucky wit 3x17, 14, 13, 10. the increases came with the ability score improvement via level ups and Crooked Moon origins also let you increase two scores, which he immediately took into charisma. (meanwhile bard got 4 in Int and druid 4 in charisma) Sounds strange, I know but on the other hand, I myself rolled once a Character with a total score of 91 via DnD beyond.

well.. so one lesson I learn from this is, to never use rolled stats ever again XD