When I got to Moonrise, my relationship with Astarion was "exceptional" and yet, even though I didn't trigger Araj's conversation, I never got the romance scene (I got Gale's instead lol). So I can confirm that for the /real/ romance scene you need to follow up with his personal quest and kill Yurgir. Then, the following night you get the confession.
I am SO VERY SAD that in three playthroughs that I've romanced Astarion I never knew of this until now. Both conversations feel so different and imply so many different things. One feels kinda wrong, like you should just end up being friends because you started off like a very obvious fake thing and the dialogue doesn't help you understand this differently. Astarion still struggles with his feelings about relationships and his past and he's nowhere near ready to bump into a serious relationship. On the other hand, the other romance scene -even though it happens right after you do something important for him- feels true and sincere, almost as if Astarion made peace with his feelings for you despite his past and was somehow ready to delve into the beginning of a relationship. It really feels like he is falling for you instead of just feeling grateful for not forcing him into some random drow.
Although both conversations lead to the same outcome, they really shouldn't, one feels truer than the other. On future playthroughs, I will never, ever trigger the npc's conversation again.
I also felt like that romance scene triggering through Araj is just... too random and too early in Act II, I very much prefer a random event led by approval or after the completion of an important milestone on that character's personal quest.
