the ending feels like a defeat
This! So much!!!
We start our journey trying to get rid of the tadpole. If we put our faith in the Emperor then half way through our brain is filled with tadpoles. And then he even offers us a super tadpole.
If we don't do it the Emperor's way... well too bad. You still have to become an Illithid.
On this matter the game wants you to put all eggs in one basket. Whereas the other quests often give you more options. Some of them including one that says we'll find another way.