Did you not get the dialogue between Gale and Wyll? It's about that they lost a lot of power since they got the tadpoles. So basically, the tadpoling pushed them back to level 1.
Plus in Gales case, he also lost Mystras favor and therefore even more power.
This just kind of reads like a hand wave to me at least. The tadpole doesn't really seem to want to take your powers away, abyssal sorc shows people can still be super strong even with a tadpole in their head, and he's still perfectly able to level up and cast normally.
I personally think the impact a backstory that big has on your plot progression is a larger issue, but it just feels a bit contrived to me that the tadpole would drain all of your levels without causing any other issues, and that it would also do this to martials like minsc or karlach too (as even if she is a paladin she's not gonna have a level 15 paladins health).
Also I'm a bit unclear on his relationship with Mystra at this point, doesn't the weave scene suggest she's still looking over him?
