I have found a homebrew Mind Flayer race* and am wondering if it is reasonably well balanced against other races.

I have removed the fluff like age etc and just left the important parts.

My personal conclusion is that it possibly on the strong side, but still remaining within the bounds of power.

Ability Score Increase.

Your Intelligence score increases by 2, and your Charisma score
increases by 1.

Size.

Most illithids are taller than humans, but their bodies are slender,
weighing much less than an average being of their stature. Your size
is Medium.

Speed.

Your base walking speed is 30 feet.

Darkvision.

You have darkvision up to a range of 60 ft.

Magic Resistance.

You have advantage on saving throws against spells and other magical
effects.

Mind Blast.

You may use your action to attack anything in front of you with a mind
blast. When you use your mind blast, each creature in a 15 ft. cone
must make an Intelligence saving throw equal to 8 + your Intelligence
modifier + your proficiency bonus.

A creature takes 2d6 psychic damage on a failed save, and half as much
damage on a successful save. This damage increases to 3d6 at 6th
level, 4d6 at 11th level, and 5d6 at 16th level. After you use your
mind blast, you can't use it again until you complete a short or long
rest.

Devour Brain.

If you consume the brain of a recently deceased creature, you briefly
interface with its mind. By doing so, you learn at least one piece of
useful information the creature knew.

Telepathy.

You can speak telepathically to any creature within 120 feet of you. A
creature understands you only if the two of you share a language.

Languages.

You can speak, read, and write, Common and Undercommon, and can read
and write Qualith.

*I can't remember where I found this, so apologies to the creator to whom I can't credit.

So, taking these one at a time...

ASI, Speed, Size, and Darkvision

Normal, moving on

Magic Resistance

Equivalent to the Magic Resistance trait of the Yuan-ti from Volo's Guide. Even on the Yuan-ti, this trait is considered overpowered.

Mind Blast

At its base, this is a reworded carbon copy of the dragonborn's Breath Weapon. However, it targets an Int Save and deals Psychic damage. Int Saves are relatively rare, and a lot more creatures have a low Int score than have a low Dex/Con score. Additionally, elemental resistance/immunity is more common than psychic.

Devour Brain

Hard to quantify. Depends on the sort of campaign you are running. But it's probably fine

Telepathy

Longer range than what a Kalashtar has, but is limited by a shared language and doesn't allow return communication. This is probably fine

Languages

Again, normal. Though as a minor nitpick, I'd note that Mind Flayers generally do not speak Common.

Summary

Magic Resistance is significantly stronger than any other race's ability to resist harm. For that alone, I would hesitate to allow this at my table. Pair this together with the built-in AoE that is a bit better than the dragonborn's, and I'd rate this race as notably more potent than average. Probably not gamebreakingly powerful unless you were in a game with a TON of magic-using foes...but certainly more powerful than standard races.