Astarion and Halsin wouldn’t consider themselves partners for a few years. I think Astarion would make more effort to speak with Halsin after Tavlin and Halsins relationship begins. Astarion perception of Halsin at that point was that he was a big horny dope, and hadn’t bothered too much to pay attention to him. Astarion was preoccupied with figuring out his feelings and fears with Tavlin as well as surviving the next day. After Cazador’s death, everything becomes more manageable, and he finds himself genuinely interested in learning about Halsin. I think they’d be good for each other. Manage to actually talk through their repressed traumas, when they have opposing feelings when thinking back on them (Halsin painting them in a brighter light than they are, Astarion’s disgusting and hate of it all). When the netherbrain is defeated, Astarion stays behind in Baldur’s Gate with Tavlin who rebuilds and revives Selûne’s church, and Halsin travels to moonrise to rebuild there. They visit each other frequently, and here I think is when Astarion and Halsin would actually fall for each other.
Astarion and Tavlin (and Halsin) are allowed to freely express themselves sexually and emotionally with other partners, though they find that the emotional aspect of a relationship is very difficult after experiencing such a unrelatable trauma. Tavlin mostly takes advantage of the openness of their relationship, since he was so inexperienced at the beginning of it. Astarion wants to, but it brings up more trauma than he would like — not being close with his sexual partner, putting trust into someone he knows barely anything about. Halsin on the other hand, is the perfect option for exploring his sexuality — and he is very enthusiastic to do so. They go from friends, to friends with benefits (i suppose lol), to finding themselves incredibly invested in the other. I think it takes a few visits for Astarion to realise that he’s in love with Halsin, because he’s only really felt it once before and it was so difficult the first time to admit that he was in love with Tavlin. I think Halsin felt very strongly for Astarion very quickly, but he was happy to be whatever Astarion needed him to be, not pursuing his feelings. They both talk extensively about it to Tavlin, and he would have a field day with it. Trying to get them alone together, stupid cliche romantic things that he had read about in his romance novels. I think at some point it would just click. They’d stop being awkward and apprehensive, and Astarion would tell Halsin that he wanted it be more.