--so what kind of consequence are you looking for? Do you want the tadpole user to be punished? How so? I mean specifically, what should the punishment be? This is a sincere question.
--or do you just want a reward for not taking advantage of power in the game? If so, I think that's fair. What kind of reward do you want, though? An achievement? A special item?
Personally, I'd like it if the more you used the tadpole's powers, the more likely it was to compel you to make further tadpole-oriented choices - positive or friendly resolution options disappearing or starting to prompt wisdom saves to succeed on saying, and even sometimes all options but the authority abusing ones disappearing, if you don't start fighting it.
Does it do this to some extent already? I’ve not played around a lot with options as I’m only on my second playthrough and still avoiding unnecessary spoilers so haven’t watched vids of other players. In my first playthrough, I just absorbed one extra tadpole, and then no more. In this one, my character used three, and then when offered the astral tadpole tried to resist but failed a saving throw and the narrator said something about her mind no longer being fully her own. It’s cast a bit of a pall over her act 3 (as well as messed up her looks) but definitely felt like a consequence to me. Though being able to fly is pretty cool

I do agree with the general point though, that tadpole use should at some point become irreversible such that even if you win you’ll never be fully human/elf/whatever again, and could find yourself increasingly struggling to take actions of your own free will rather than doing what the Emperor wants. Though to really feel truly consequential, there’d need to be a good alternative to turning mindflayer for someone who
doesn’t chomp tadpoles and, has been commented elsewhere, that feels somewhat lacking at the moment.
(More fundamentally, I’m not a fan of the tadpole mechanic in it’s current form at all, and probably would have preferred it if it hadn’t been introduced. But given we’re stuck with it, I’d be all for some changes to make both using and not using tadpoles more of an interesting choice.)