Rote90 you got me thinking, I rewatched my Tav's encounter with Mizora, the Graveyard scene with Astarion and the Patch 7 Youtube video of the added content for Spawn Astarion. And I do think the Patch 7 added content is in alignment with his charcter IF it is post graveyard scene. My Tav said "You are the person I want, the person I love" and Astarion says "I love you, I love this and I want it all" (I think I got that right or close to) and at this point I do feel a deeper commitment is made and a night with Mizora after this is a serious betrayal of trust.
However, my Tav's encounter with Mizora was back in Rivington, pre Cazador and graveyard, at this point when asking Astarion about the relationship he says "I don't know, but isn't it good not to know" to me, the night with Mizora is still cheating but at a far less extent, as the boundaries hadn't yet been firmly set on the relationship or the deeper commitment made. I am curious now whether Larian has made any distiction between pre or post graveyard as well.
I know not everyone will agree with me, cheating is cheating, I would agree that is true for most other companions as their boundaries are very clear.
In my Tav playthrough, the night that Astarion invites Tav back to his bunk, he says "you'll be all mine and I'll be all yours for tonight at least" and then Tav finds out the next morning that he also had sex with Laezel that night (Teifling Party), so for my Tav, Astarion is a bit of a player.
I like my Astarion firmly in the grey, Spawn or Ascended. I don't see him as being really good or really bad. To me the core difference between Astarion's development paths is that Spawn Astarion chose freedom and Ascended Astarion chose power.
I also agree with you that Astarion is learning to be a better person, however his good or bad development is heavily influenced by Tav, especially on the journey leading up to the defeat of Cazador.
BG3 is such a great game that it can create so many stories and that it is so much more than just smashing keys and killing things. I love that it is a mix of strategy and story.