I ended up with Astarion. I was en route to romancing Shadowheart, but I got surprisingly few approval moments or interactions with her throughout act 2. At the same time, my character seemed to be taking the shape of "goodie-two-shoes but sassy bitch to those who deserve it", which gave loads of Astarion approval.

I ended up learning into it and I'm super glad I did. There's an Act 2 Dark Urge scene that I felt completely changed the romance dynamic between my character and Astarion - it felt like it put us on a much more even footing, and I was surprised by how sincere and empathetic the scene felt (given that Astarion's main Cope is a mask of flirting and sass).

I had originally wanted to romance Karlach, but there was a bug that prevented her approval from rising and by the time I discovered and fixed it, I had missed loads of Act 1 approval moments