WinkVibe

Encounter Building: I'd consider general flow of the fight & what you expect to change during combat (e.g. reinforcements, thralls being freed, ulitharid vs elder brain tension, defensive perimeter, intellect devourers springing from thralls), but I'd start with the elder brain, ulitharid, and 4 mind flayers as the heart of your battle and build around them. Then sprinkle on other lesser foes as needed/desired.

Creature Sense & defensive prep: Elder brain can sense creatures up to 5 miles away, so unless the players are taking special measures or have some plot mcguffin, the elder brain knows the party coming and should pursue additional recon (e.g. cranium rats) and make some defensive preparations.

Sitting Duck issue / encounter distance: In the face of artillery/blaster superiority – which your party definitely has in spades – the elder brain could be a bit of a sitting duck. Definitely I'd set up the terrain/room so that the furthest a creature could be from the elder brain & still have line of sight to it would be 60 feet. In fact, since it's only reliably dangerous at 30 feet, I'd consider some kind of cover (or even a magical field or mucilaginous sphere) surrounding it at 30 feet away, so if you're outside of that area it's harder to affect the elder brain, but once inside (and thus within range of its tentacles) it's a normal attack/save.

Cavernous terrain: I would look for an opportunity to showcase what is unique about the underground terrain in this particular area, and ideally make it advantage the illithids (at least initially). Something like "scream stone" formations which if it would take psychic damage emits a high-pitched magic negating/draining pulse.

Exploitable echoes of the githyanki fight: Some way for Channel Divinity or Spell Slot expenditure to activate astral energies of a device, trap, or portal left in the wake of the githyanki assault. Ideally in a risky position. Something which, if the players think to use it, take the risk, and spend the action in combat, could give them the upper hand (e.g. teleporting past illithid defenses OR escaping the illithid's stun-locking potential).

Illithids vs. Tanks (Monster Tactics): The regular illithids should prioritize dominating the barbarian, and using Tentacle attacks vs. Cleric (or really anyone except the Bard and Wizard). A 19 Intelligence foe should be able to deduce this strategy within the first round of combat, or perhaps even before combat if they've been apprised by the elder brain of the party's composition. The battle with the regular illithids will be the players balancing using their action economy to keep distance between themselves and the illithids (as illithids are scariest in melee), vs. using their action economy to target the elder brain and/or ulitharid.

Thralls... use with caution ...possibly tie to Legendary Resistances: Since your party was just dealing with a murder scenario, IF you want to use thralls, I'd suggest building in some way to free them (or at least the most sympathetic of them) from the illithid control. Of course, players may want to turn their new-found allies against the illithids, so you might want to foreshadow a way to break the thrall-dom but temporarily incapacitating the victims.

I'd also consider giving its Legendary Resistances a concrete embodiment in victims trapped within pools of green gel that can be drained by the elder brain's tendrils. This creates a little dilemma / mini-game in which players can either try to free or kill these victims to deny the elder brain Legendary Resistances. Just a thought. This approach is pretty group dependent, but we've had fun with it.

Ulitharid's motivation: Since the ulitharid is the closest to a mastermind NPC in this scene – the elder brain being more monstrous – I'd ask myself why is it working so closely in tandem with the elder brain? My understanding of the lore is that there's usually tension there as the ulitharid is always one step away from pulling half the colony away from the elder brain to create their own underground empire (and eventually become an elder brain themself). More a lore-building question, but if you decide there is some tension there (e.g. maybe elder brain blames ulitharid's risky ambitions for recent githyanki attack), that might be a wedge the players could drive between illithid forces.

Stun-locking: I hinted at this, but with multiple INT saves risking stunning, it's very possible you will have half the party stunned each round. From my view, that's your biggest issue to design around. That can be a real drag, and I'd consider how you want to play that. One way I've gone is to create a sort of psychic dreamscape which stunned character can explore, allowing them to learn some lore and potentially discern weaknesses of the illithids. Another way I've gone is to provide a limited use resource (icky consumables called "psychic leeches") that counteract illithid stunning for a short duration like 3 rounds, but afterward you suffer 1 round vulnerability to psychic damage. Or if you have a highly tactical experienced group, they might factor this into their planning and devise their own countermeasures / adaptive response.

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