In my opinion if they want Halsin as a proper romance partner they need to rewrite him so that it starts in act 2 (or even during the party) already and remove the poly. He can still be dedicated to cleansing the shadow curse similar to how you can start romance with Shadowheart while she's still dedicated to her mission for Shar.

Right now you can't start (properly) romancing him until act 3 at which point all other companion romances are already ongoing or stopped, which essentially forces it to be full speed ahead since you're already near the end of the game and has to be poly unless you rejected the rest.

Gods, just imagine Minsc was a romance partner. You recruit him and the first thing he'd do is ask you to sleep with him because you might just end up going straight for the final battle a minute later.