lesser magic resistance is literally gnome cunning, and pureblood yuan-ti gets full magic resistance on top of delayed innate spellcasting and poison immunity.
the power level shouldn't be the issue, they rank about as well as elves and dwarves in feature strength, while not being as stupid as yuan-ti. i rationalize monstrous races as being able to be better because fitting them in is harder and they can't enjoy any of the normal social interactions common races can, so maybe you are better at murderizing or surviving in the wild, but are absolutely awful at public presentations and lawful negotiations.
as for the brains, the fluff text is not final (i kind of stopped caring about it and just put some stuff from volo's) but yeah, brains. i obviously made my research and it's not super harsh as you'd expect, at least for a solo mind flayer.
pic related from lords of madness, you still eat normal food and only really need a humanoid brain per month to survive. you could easily manage with shittier brains like goblins, orcs, grimlocks or kuo-toas, but i'm sure no one would miss the town's drunk if we only stay the night...