I am going to laugh if those who indulge in the tadpole mastery end up becoming Mind Flayers at the end.
If I can still play as a mindflayer I'd take that as a win. Larian has said that we can embrace these powers risking our soul to save the forgotten realms or doom them...heavily implying there is choice, that you still have agency and the ability to act after whatever consequences or transformation, and that a good end may still be possible if you are so inclined. I think it would be very interesting to turn into a mind flayer that has all the memories of the host and isn't bound to an elder brain and still has agency to act...that is the greatest fear of the Illithids...their own nightmare and boogieman, they call him the Adversary. Would be very fun to see that happen in BG3.