The ‘thank you for trusting me’ line you get the morning after being turned always felt sincere to me, as opposed to one of his sweet nothings. That whole conversation reads as him finally breathing a bit easier and getting to revel in his evil plans with faith that Tav wants to share in them fully.
I also found it jarring that you can tell Jaheira in the epilogue that Ascended Astarion is the love of your life (and it’s not flagged as deception) but you can’t tell him that. Mind you, Jaheira will also tease you about having children in the same conversation without seeming to realize that two undead are probably not going to procreate.
