But I'd echo what Rote wrote above in that it's a shame that most interesting interactions with him are locked behind possibly relationship breaking actions while there are few affirming actions that provide additional narrative.
I'm never sure if I feel this way as well because I'm a completionist, but I've always held his romance flows way more naturally if you're romancing someone at the same time and their act 2 scene triggers first, making Astarion talk to you about it. I think it adds the step that's missing behind his attitude in act 1 to his attitude in act 2, I found the shift a bit jarring when the only other build-up I can think of is the scar scene (which is greatly done! But I do wish there had been more exploration in-between)
I completely agree (and I am so sorry Wyll) Especially if you trigger the Act 2 scene through Yurgir, a long time passes between his act 1 scenes and his revelation, but even if you trigger it through Araj, it feels like a step is missing. While I think that both the scar scene as well as the scene during which he gets jealous are nicely done, I am most surprised that the scene about taking over the cult does not have a romantic variation in which he tries to tighten his hold. Controlling the tadpole is, after all, one of the two reasons he might need his powerful ally for.
(I hope it's ok we got slightly off topic.)