There are plenty of cool races out there. The generic one that everyone loves is the human (variant) as you can pinpoint two +1 to ability scores and gain a feat.
Other nice ones that I've seen are Hobgoblin if you want to play a disciplined soldier. They gain bonuses to CON and INT (your main 2 stats) and proficiency in a couple of weapons if you want to keep that melee presence up.
For more HP and tanking, the Dwarf is pretty good at this. You gain even more hit points per level if you go Hill Dwarf, effectively letting you become a damage sponge. Mountain dwarf (while the +2 Strength mostly goes to waste) gains medium armour proficiency, allowing for better defensive capabilities if you're not pumping CON all the way up.
The Half-Elf has the jack-of-all-trades capability like human. Good ability score improvements, a couple of extra skills (or swiping an elven racial ability) works really well.
I have a tiefling mystic, and they're great to play. Their racial spells help cover some areas (a counter-attack spell in Hellish Rebuke to punish getting hit, Darkness to cover yourself temporarily, etc.), as well as good stat distribution. The variants of tiefling all have cool reasons, too, but vanilla tiefling works just as well.
A strong overall one, like human/half-elf, is the Warforged, specifically the Envoy. Choosable stat bonuses, extra skills, available AC bonuses (if you're not keen on the CON reliance) amongst the general robot-powers.

To say the least, there's plenty of cool ideas for which race to pick.

This entirely matters on how your game is played. If you roll for abilities, then you could mix and match how you see fit.

Intelligence is the main ability score for all your psionic disciplines. However, there are plenty of disciplines that do not rely on saving throw or attack rolls that means you can keep it lower than usual. I did a build of an INT 10 half-elf mystic soul-knife, and it worked quite well. However, with Order of the Immortal, it does boost your temporary hit points per round at 3rd level, so not one you want to dump.

Dexterity is possibly one of the best ability scores to play with in general for D&D 5e. It affects AC, Initiative, a lot of spell-based saving throws, attack rolls with ranged and finesse weapons, a good chunk of skills.

Constitution is something you never want to dump. Especially for an Order of the Immortal. It affects Hit Points, Concentration checks (which you will need to make!), resisting poisons/diseases, etc.

Wisdom/Charisma are less prioritised for you, however are both useful. Not only for those specific saving throws against nasty spells, like Banishment, but they affect a good amount of skills.

Strength is rarely needed for a mystic. Unless you want to physically "tank" with a sledgehammer on the end, it's probably not worth it. You can use your talents to hit people pretty hard, anyway.

In short, I'd go for Intelligence as your primary focus, then Constitution and Dexterity. If you're not bothered about your spellcasting DCs, then put Intelligence in a third place priority. The other three are less vital.

Beacon: If you don't have Darkvision, might be useful. If you want to attract attention, can be used to draw opponents over to the big glowing person.

Blade Meld: Outside of very specific cases, you're not going to get disarmed. There's one PC ability (battlemaster fighter) that can potentially pick disarming strike, and a couple of monsters in the books that gain this ability. Not really worth it.

Blind Spot: If you're wanting creatures to come attack you, why are you making them not be able to see you? they'll prioritise your opponents at that point.

Delusion: If you like playing tricks with Minor Illusion, it's a fun alternative.

Energy Beam: It's an okay ranged alternative.

Light Step: Useful for getting that extra bit of movement if you don't have Celerity or something similar.

Mind Meld: Depends on how you play your game as to how talking across to players works.

Mind Slam: Quite nice for knocking adjacent enemies prone to keep them near you. However, it is a CON save, so physically bigger threats are less likely to be affected.

Mind Thrust: an INT saving throw cantrip? Pretty nice for damage, but that's not our focus right now.

Mystic Charm: It's friends

Mystic Hand: It's mage hand

Psychic Hammer: Like Mind Slam, it's battlefield control, allowing you to move a creature 10 feet in ANY direction. Towards you, away from your allies, it's pretty great to control the battlefield at no cost (beyond your action for the turn). It's a STR saving throw, so not great on physically bigger threats.

There are a LOT here. Let's just point out some things that might be useful:

Adaptive Body: Great for resisting elemental damage or environmental effects, so it's very conditional tanking.

Bestial Form: Tough Hide is +2 AC for 1 hour, which stacks with your AC calculation. The rest are nice for adapting movement in the future, but not vital.

Brute Force: Only really useful if you're going full melee and smashing people with melee attacks.

Celerity: Good for battlefield movement, dodging as a bonus action. Basically it's multiclass monk.

Giant Growth: It gives you some temporary hit points, but this doesn't synergise with your 3rd level archetype feature. It does make your size bigger, meaning more creatures will be able to get to you, or you can block a narrow passage, etc.

Intellect Fortress: Nice for against spells that attack mental stats, and also can use 2 psi to give disadvantage against you. Makes you more of a dodge-tank.

Iron Durability: +1 AC? Nice. Scalable AC bonus? Also nice. Barbarian resistances? Very nice!

Mastery of Force: Once again, battlefield control to keep things near you or grappled can be very useful. It also has Inertial Armour if you don't want to focus on CON for AC (and can be used with a shield if you end up using shields)

Mastery of Ice: 20 temporary hit points for 3 psi? It's like pre-emptive healing.

Nomadic Step: Early teleportation is great. Means you can be where you need to be for tanking.

Psionic Restoration: Heal yourself? Able to remove status effects for zero extra cost? Count me in! I always end up picking this up.

Psionic Weaponry: Like with Brute Strength, only has any benefit if you're planning on hitting them with a stick.

Psychic Assault: Psychic damage is hard to resist, and one way to draw in attackers is if you aggro with annoying psychic blasts.

There are plenty of others, but these I feel had something to comment on. If you're wanting to be in the fray and stopping getting hit, I'd say Iron Hide and Psionic Restoration work great as your bonus disciplines.

So there aren't a huge amount of feats that narrow down to what you want, but here's some good ones:

Alert: +5 Initiative and go in surprise rounds means you are more likely to be where you want to be at the start of combat.

Defensive Duellist: Effectively a mini-shield at-will if you're holding that trust dagger.

Lucky: Always good. Having three rerolls a day is amazing!

Marital Adept: If you do go down a melee hitting build, having a couple of manoeuvres available to force opponents to do thinks (Goading Attack to be more likely to attack you, Parry to reduce damage) there's some uses with these.

Resilient (CON): Proficiency in concentration checks and potentially more HP

Sentinel: This is a great tanking method. Disadvantage as a reaction if they attack someone else, prevent them from getting away, all in one comfy feat. Better if you are decent with a weapon.

Tough: More hit points is great.

There's a lot of items that can be useful. Keep in mind you do not have the spellcaster keyword, so a lot of magical wands, staves, etc. can't be used. However, there's some things to think about.

Finesse Weapons: If you get a bonus proficiency in martial weapons (from race etc.) then things like the rapier allow you to have a decent melee attack, which is more of a threat then using that flimsy dagger.

Cloak of Defence/Ring of Protection/Bracers of Defence: Given it's likely you won't be wearing armour, giving yourself something that statically increases your AC works.

Rings: There are so many rings that are useful. Ring of Free Movement to ensure you can't be held, letting you get to anywhere you need to tank. Ring of Spell Storing allows you to take a couple of spells from your allies to dabble a bit in specific castings. The legendary rings are fantastic, too.

Mystics can't use things like the wand of the war mage, nor a magic weapon, to buff their spellcasting unfortunately.

Amulet of Health: Don't want to pump all those ability stats into CON? Well just get it to 19 instantly!

Belt of Dwarvenkind: +2 CON for using it is great (+1 HP/level effectively) and it can give you manly facial hair.

Cloak of Displacement: Gives you a body double, mainly forcing disadvantage nearly every turn!

Gauntlets of Ogre Power/Belt of Giant Strength: If you wanted to hit hard, but still wanted to dump STR, then here you go!

Headband of Intellect: Yada yada 19 INT

What this is quickly coming down to is that Wondrous Items are where it's at. You don't need armour, your weapon pool isn't great, and the spellcasting implements don't work with psionics. I'd advise talking with the DM to see if there could be some homebrew magical items that mean your mystic can have equivalents that other classes do for empowering their attacks etc.