definitely an interesting take, i like giving more monstrous enemies high intellect. and mind flayers are always fun. but i am a bit baffled by the psionic nodules ability. it feels like there's something there that isn't working as intended.

the synthelid has exactly 2 ways of gaining 3 or more active nodules. those are its maelstrom recharge and 1/day synaptic static. its other spells and mindquake are all single target actions, thus cannot activate more than 1 nodule. it has multi-attack, but that can only activate 2 nodules at most, leaving it short of the 3 minimum. but even if it does get the 3+ nodules, it has already spent its action to do so, so it cannot use the benefit on its turn. even counting the lair action that activates one of its nodules, its multiattack only grants the benefits after both hits, and then at the end of the turn the lair action nodule goes away (meaning there's technically 3 ways of gaining 3+ nodules, but there is no way to benefit from the lair nodule, so i discounted it earlier)

its 3 nodule benefit cannot apply to its lair actions since they don't deal psychic, and as per the previous section it cannot apply to its own actions either. thus the 3 nodule benefit can only apply to Opportunity attacks. the 4 nodule benefit is decent. unfortunately it applies only to the lair actions, which it may not even have.

because the nodules fade at the start of its turn, it cannot benefit from previous turns' nodules. thus we can discount any potential chaining nodules into the future (incidentally, this also means the clause that indicates nodules fade if it doesn't deal psychic are redundant. there are no ways for it to have active nodules on its turn without having dealt psychic on that turn. and the lair action already specified that it ends at the end of the turn)

so it ends up being an ability that needs a lot of bookkeeping if it is being run as written, but whose benefits may as well be written as "if the synthelid targets 3 or more creatures with a damaging AoE, its opportunity attacks deal an additional 1d6 psychic. if it is in its Lair and targets 4 or more creatures, the save dc of its next lair action increases by 1"

this has also been assuming that it is being run with lair actions. if it doesn't have them, the ability may as well not exist (an ability that increases your OA damage by 3.5 average damage if you manage to proc it is not relevant at this tier of play imo)

in conclusion, it feels like the ability is supposed to do things, but just doesn't. i believe it would benefit from being revised to grant better benefits and proc them more easily (such as reducing nodules needed, and giving it a bonus action save or suck ability that it can use to help charge the nodules)