I love talking about Astarion, so I'm going to jump in again if thats okay.
I did basically the whole game with Lae'zel, SH, Astarion in my party (sometimes Karlach) and I feel like Astarion is a character that I don't find hard to please the way I play while also keeping the others happy (I was friends with all four of the others), but there are some approval boosts that it helps if you know they are there. For instance, if you pick him up on the beach, rest immediately before picking up any other companions. In that dialogue, there is an approval boost for saying 'thanks' when he offers to watch over the camp that night lol. When he offers recommendations for how to kill you, and you respond 'poison' as I did, he also approves and I think he approves with the other method options too. Also, there is a one-off dialogue with a cat where if you talk to the cat and admire the cat you get two approval boosts. + neither of these effect SH, Lae'zel or anyone else. (Basically, you just have to spend time with/on Astarion). Also, if you make Zorru bow to lae'zel and then choose to wait for him to do it, there are approval boosts for Lae'zel + Astarion, and in the following conversation, another approval boost for L + A but a minus for SH. There are also L + SH boosts that don't effect Astarion at all, so all of these end up balancing themselves out more or less).
If I were to tie that into this conversation I'd say, it is impossible to sleep with Astarion by or before the second long rest, except for maybe if you are just not long-resting, and even then that will still be hours into the game. First long rest w/ him you either have a dialogue or someone else's welcome-to-camp dialogue, second rest he stalks off into the wilderness, third rest is the
(which can be pushed off further if you have other cinematics to see first or something). The dialogue where Astairion is looking at the sky is tied to his 'fair' friendship and I usually get around Druids Grove, but no matter what you choose in this dialogue, even the 'im interested', you can't sleep with him here. He refuses you. Then, at 'good' approval, at some point when you start a conversation with him, he will proposition you and then at the next long rest (as long as there are no other cinematics waiting) you begin his romance then. Otherwise, it's after you do the druids grove quest or goblin quest.
Other cinematics with other companions, and story beats, all push these things further back.
Visiting druids grove does not have enough approval 'ups' to get you to 'good' approval. Or, at least from what I've seen. The soonest ive ever started a relationship with Astarion was at the goblin camp and hours into the game. So you would have to be speedrunning friendship with him, and taking him everywhere with you, and never long resting, for him to sleep with you by second rest.
Despite being flirty, he doesn't throw himself at you, and from what I've heard, and the statistics Larian gave out, people actually get rejected by him very often.
But if you are even interested in Astarion a little I would recommend you at least take the time to be his friend. He deserves nice things.
