Hello to everyone here!
As a fellow, who romances Halsin, I just wanted to share some thoughts on this romance and, thankfully, there is a topic on this forum about this.
First of all, English is not my native language so I am sorry for any mystakes I can make.

I am a person who likes romantic options in games, especially I like romances with "good guys". I haven't played BG3 much when it was at EA stage, so I basically met Halsin, spoke to him a bit and that was it (in my playthrough goblins attacked Grove, I haven't foung him in their camp, I was just "touching" game and decided not to stick my nose everywhere so I had no idea where he might be. So Halsin appeared after the attack).
I knew he is from "good guys" team and I heard he is polyamorous/polygamous (I personally am monogamous). That didn't stop me.
Currently I am stil in a proccess (Act 3), I am romancing Halsin and only him and on one hand I am in love with this character and I feel he is in love with my girl Tav ( I had sex scene with him and I just can't stop myself from a small ritual before going to ben in camp: pet Scratch and baby owlbear and kiss Halsin goodnight). But I find his romance progression to be messed up at some points (yes, I understand that writer(s) had to work fast with him because of all the love Halsin got from fans).
So, these are my thoughts:

1. I think his romantic/sex scene and his confession should be moved to Act 2 (end of act or early Act 3). I am a patient person, I was ok with waiting for Act 3 to at last be with handsome druid, but still I think things should happen sonner.
2. His dialogie about polyamorous/polygamous/fluid relationships must happen in Act 2 before romance, when we learn more about him (his family, his love towards ducks, etc.), so a player can be ready that this one guy can be with more than one partner. It was akward to learn about his preferences when you hear him confessing his feelings to Tav: "Hey, I want to be with you, but also "bla-bla-bla about wolves and bears". It felt like an avalance. When I was playing I didn't ask him about such stuff just because at this very moment I, as Tav, wanted to be with him. For other players, who decided to ask him other questions, this dialogue option, I think, was not very pleasant, especially if they didn't know Halsin is into poly.
3. We need to have options for monogamous people in his dialogues. For example, when Tav is already in relationships with him there might be a discussion about this and, depending on decisions or something else it would be nice to have two ways: either Halsin remains poly and Tav is ok with this (if Tav is not ok with this than, I guess, it might lead to the end of romance) or because of some dialogues, actions, whatever, Halsin realises he wants to be only with Tav.
Now, there is a moment: during his confession Halsin says he want Tav to share their heart with them. When we speak with Halsin about relationships he says something like "You are all I want but I don't want to tie to myrself". I think we need, as I said before, options and changes of these dialogues. Halsin respects Tav and want them to be happy and if they are in poly relationships he can say what he says now about sharing happiness, and if our story leads us to him accepting monogamy this is where he can say" You are all I want, and no one else" or something like that.
4. Story of him in the Underdark *may* have some extra dialogues at camp before sleep if, for example, Tav is drow or half drow. In my headcannon my Tav (she if a half drow) feels horrible after learning such detailes and feels some *responsibility* for actions of her kin. She may come to Halsin and share her feelings that bother her, and he may tell her that he processed these events many many years ago, he doesn't feel traumatized anymore and puts no blame on Tav, and then he comforts her and everyone is happy.
6. Halsin's pipe. It needs to be included in romance with him (and maybe not only). Something like "Oh, I got your pipe! I didn't know you are a smoker, tell me about it". Or he can tell about his pipe he left in the Grove and Tav can buy him new one. Plus, I think it would be nice (not only for Halsin but for others) to have an option to make gifts for your love interest and for them to make gifts for us too.
5. As I said, I am still playing my firts playthrough but I already heard that epologues are complete mess and romance with Halsin has no logical ending, so yeah, it must be fixed, as everyone said many many times. I heard something like that happened to DOS2, so let's just wait.

well, here are my thoughts.