I am not against a monogamous Halsin romance - it is probably what i would play if it was a possibility. But I think you aiming for that is a high ask now that he ask been introduced as he has.

Being RA means that you can be polygamous or monogamous. I am monogamous RA. In Halsin's case this means that he will always evaluate the relationship. And that's a healthy thing. This compromise actually allows you to have a monogamous romance with him, where he accepts the choice, but also says that it is something you should leave open for discussion later on if needed.

The rest is your personal headcannon from there on. So, if in your head cannon they're so happy and content with just each other, then that is exactly what will happen in this type of relationship.