I agree with most of what has been said and I'm glad @fylimar raised the topic.
Yes, there needs to be consequences for tadpoles. I've not used tadpole powers except for the railroaded "you need to have or be a mind flayer" at the end but I cannot agree that the powers are insignificant. Black hole gives you insta kill with a great many enemies, a free counterspell that automatically levels is incredibly powerful and fly is nothing to sneeze at.
I like @Niara's suggestions! I would add having reduced wisdom saves to The Guardian at moments that you defy it. At present it only expresses annoyance or contempt when you have the Orphic hammer. While it claims it will never let you into the Astral Plane it decides to anyway and lets you get right next to Orpheus hold the hammer before it says anything. Which is pretty dumb even by end boss standards.
(I also think the fight with The Guardian should start right away but that's a different discussion)
Other places will checks would be appropriate:
* When recruiting Minsc. Of course let Jaheria save us at the last moment but I think the story would improved if you saw how the tadpoles were sapping your will
* When speaking to Voss in the sewers
* When walking into Ansur' lair
It would give the "you are my puppet" moment more gravity and make the moment when you free yourself from the leash of the gaslighting ghaikboss feel like a victory.
Other ideas:
* Require clerics to atone. (ceremony spell with some sort of sacrifice - magic item or gold) In Larian's interpretation of mind flayers, illithids are soulless. Clerics are in the business of getting souls to their god's afterlife choosing to inch closer to becoming a soulless being "devoid of hope and conscience" has got to irritate your god - especially after you discover that tadpoles are integral to another god's evil plot.
* Have Withers remind you of answer to his question - if you thought souls were priceless why are you chewing away at it in hope of gaining more power?
* Reduce persuasion checks and heighten intimidation checks. Because eldritch horror eyes tend to put people on edge.
