the illithid retains their personality
That is not entirely corect tho ...
Illithids are DnD response to "ship of Theneus" (google it, its fascinating thought experiment).

Illithids copy your whole life, personality, experiences and thoughts into its brain, and while it may seem like that person is still there since that is all they contain in moment of transformation ... the original person died in the process and this is entirely new being ... ergo
(we see this in the example of Karlach, who changed a bit, but retained many aspects of her personality)
And it will dissapear bit by bit as the time goes ...
SInce the same "copy all" process will go with every single feeding.
As the matter of fact, our protagonist should have get game over, once we decide to turn.

But that would be boring.
Those who have been forcibly made illithid will naturally take revenge
Agreed.
They may do what you need, but they should logicaly seek a way to mess you up in the process aswell.
the number of tadpoles is not important, one tadpole is enough
+1
But there could be more volunteers
Exactly!