my sad little druid had the worse run of romances and left Act 2 single and as Halsin was saved I at least had an option.

From a perspective of Halsins character in act 3 I don't really think he falls into the category people are putting him in .

The drow scene I will never touch though I have the discount due to the murder trial
Halsin says clearly all he wants is you but feels wrong about denying others the joys of being with you .
repeat "All I want is you "

the impression I got from before was that he was offering /forcing Tav into the multi lover situations abrasively I turned him down on previous Tavs as I tend to mess about with companions Act 1 then settle down in Act 2 usually with Astarion who is one of the them that says yes (though I don't believe he's happy about it) I still will not go the threesome route.

I think they could tidy it up a bit if Halsin ended up being your only romance his kiss scenes are sweet and I like him calling me "My heart" and asking for another kiss instead of constantly repeating can I kiss you .
they will indeed need to write in some solomance content however I think the voice actor has said its over and done with.

Personal wish list :
In thoughts and dreams they could put in the option to share a drink with him rather than him shooing us away in act 1 , act 2 once Oliver has returned to Thaniel could really be a trigger for a cutscene, Act 3 we can discuss saving the refugees and take them back to the shadowlands to repopulate the place say use Yenna for the trigger if you save her from Orin.

However maybe you need him to be an ACT 3 only thing maybe the developers thought what if Idiot Tav messes up all relationships in act 2 and are now feeling lonely give them an option .

not necessarily Halsin related just a comment regarding all companions.

I kind of fall back on 2nd chance romance for act 3 all those date scenes will pop up in act 3 giving you a chance with your highest approval companions not all maybe say 3 of them after the emperor reveal.