I'm very glad to have an actual answer for this. But I won't pretend I'm not a little peeved that going into the game with the sole intent of romancing him can fail so easily. It does feel like I needed to meta game just to make sure I hit the proper targets tbqh.
Like, I GET it, there's obviously a lot of value to be had in a character that requires you to work hard to earn their trust before opening up to you or getting involved, but re: the whole "you don't have to bend over backwards" thing... I mean, clearly you do. It requires at least two playthroughs, and/or managing your party so that Astarion isn't literally present for any good deeds despite you doing them anyway, a knowledge of when certain cut offs happen, or a bit of meta gaming and save scumming (to figure out WHICH sassy or self serving options out of all of them will net approval, because it certainly isn't every time you snipe at someone, or ask for a reward, or similar) or, barring any of that, playing an entire second round of a 100+ hour game.
I DID check right after the Teifling party by the by. (Like, "haha can you imagine if I'd already locked myself out of it? That would be a questionable choice. ...better just make sure".) Tried to look very quickly into the Astarion romance to make sure there were other opportunities to romance him and see his full story (I have feelings on that idea in rpgs in general tbh, though needing to have a romance specifically, in order to see his full story is especially relevant with Astarion, and more understandable then usual, given that sex, bodily autonomy and romance are actively vital to his character and past, and you miss out on digging into it if you're not personally involved) if you didn't immediately start one up super early in the game.
Obviously I didn't want to look too deeply into it because of spoilers, but I came away with the understanding it could start later (and thought it might, honestly, since through cultural osmosis I knew already that past abuse and a complicated relationship with sex was a significant part of his character.) but apparently I could have looked harder. (And I can't say I have even the first clue when specifically a romance would have triggered in act 2. Didn't occur to me that I was missing starting anything there. Lol.)
Like, this game IS amazing, but I stand by my conviction that you DO have to bend over backwards to romance Astarion. The question for me is more " whether or not that's intentional, does that choice work in a 100 plus hour game, or is it more frustrating than it is a worth while statement about the character?" to which I don't have an easy answer. Personally older folk I've worked with in game design myself are always on about what a fools errand it is to build any content that assumes the player will be doing more than one playthrough of even a 15~20 hour indie. So that's colored my opinion on it, for sure, even if I don't generally agree with that sentiment.
Ah well. My experience isn't universal, obviously. To me at least, it feels like the game is asking for either meta gaming, or a time investment of multiple playthroughs in order to make his romance possible, whether intentional or not. Even if you've gone into the game with that express purpose. I'm not saying it's a game killer, obviously! And I wouldn't even bother nit picking this deeply into it if this weren't literally game of the year.
If anything, imo, it opens up a really interesting discussion about how the whole thing is handled from both a game design and character writing stand point that I'll probably be having with other people for years to come at least, which is very valuable in and of itself. =o But I'm still a bit disappointed.
