I disagree with your last point. Yeah, you convince him to not blow himself up, but for my Tav it amounted to “we’ll find another way where you don’t die.” He does get pretty power hungry, but a few persuasion checks and he realizes he was backsliding and agrees with you to cut that shit out. He decides to get the crown for Mystra without seeking its power and she agrees to heal him.
There’s redemption there, you just have to push him.