Some balance, mostly character ideas
Remove the Telepathy, style the character as the result of a failed Ceremorphosis where the character was unable to connect to the Elder Brain, and was hunted by the "hive", having to pick up the Rogue level to get away.
Make it able to subsist off of regular food; the brains, instead of vital nourishment, provide the temporary hit points you suggest (good call). Add in tentacles, without the damage, for grappling/eating purposes (and flavor):
Melee Weapon Attack: +7 to hit, reach 5 ft., one target.
If the target is Medium or smaller, it is grappled (escape DC 15).
Melee Weapon Attack: +7 to hit, reach 5 ft., one target already grappled by the mind flayer.
Hit: The target takes 3d10 piercing damage.
If this damage reduces the target to 0 hit points, the mind flayer kills the target by extracting and devouring its brain. The first time the mind flayer devours a brain after a long rest, it gains a number of temporary hit points equal to the number of hit dice of the target.
My advise is to have the original human's base personality still existing, just memory-wiped and convinced by the Illithid tadpole that they are an Illithid. The tadpole itself could even be dead, after the failed Ceremorphosis. That way, the base personality could "guide" the Illithid memories into adhering to a non-Lawful Evil behavior and e.g. avoiding snacking on sentients.
Note that an Illithid which doesn't eat sentient brains can't use psychic abilities, i.e. your Psychic Blast. (Well, at least according to 3.5e Lords of Madness)
Speaking of that Psychic Blast, consider restyling that as an Innate Spellcasting ability of the Mind Sliver (credit to Jack) cantrip:
Innate Spellcasting: The Illithid innately knows and can cast the Mind Sliver cantrip as a 1st level Warlock. Its innate spellcasting ability is Wisdom.
That way, we're firmly in the realm (here, at least) of what can be expected of a 1st level character.