I've romanced Astarion twice, once as Tav, once as Dark Urge. I have also finished origin Astarion on Tactician mode.
I play a good Tav, and you do not need to bend over backwards to romance him. I actually feel if you messed up the romance the first playthrough, play the game again as Dark Urge. Astarion x Dark Urge, is a beautiful match. You'll see a side of him, where he's very worried about you and cares for you.
Here are the important things for Astarion Romance. I am going to mark things as "Mandatory" and by that, I just mean it will increase his approval the most. Things I mark as "optional" will increase his approval faster than one-liner approvals, but are not necessary.
Act 1- Mandatory: Let Astarion bite you (this should get your approval to "Medium")
- Mandatory: At the celebration party (doesn't matter Goblins or Tieflings) spend the night with Astarion, make sure your approval is at least MEDIUM.
- Optional: Give him the Necromancy of Thay book
- Optional: Kill the Gur
Act 2- Mandatory: Help him with his personal quest. Speak to Raphael outside the Mausoleum or at Last Light Inn. Defeat or Persuade the Orthon.
- Optional: Defend Astarion when speaking to Araj at Moonrise Towers
Note: I find Astarion's confession much better when you do not force his triggers. I usually skip speaking to Araj at all. - Optional: Talk your way out of battling Thisobald Thorm. you get about 4-5 approvals from Astarion.
Act 3- Mandatory: Help Astarion defeat Cazador Szarr
- Optional: Take the Dryad Love test, gets 3 Astarion approvals if you answer correctly.
Note: End of Act 2, I am already at Exceptional rating with Astarion so I don't force him to do this.
Most important: Make sure you long rest and speak to your companions! Even if they do not have exclamation marks over their heads, sometimes they have other things you can speak to them about.
I'm doing an Honour mode run right now and am trying to romance Gale, but I have gotten Astarion exclamation marks and cutscenes back-to-back for the past 3-4 long rests. So far I've gotten the scars on his back cutscene, necromancy of thay cutscene, "You
will come to my bed tonight, won't you" dialogue, the mirror cutscene, the shallow praise/"I love you" dialogue and I'm still in the underdark :|