I just think the whole poly thing needs to be Tav initiated; when Tav is already romancing someone else as is most likely by act 3 and he be open to Tav romancing someone else while with Halsin. If Tav is solomancing him the whole poly thing doesnt even need to be brought up, especially since Halsin seems very commited to Tav regardless.

Halsin says he only wants Tav anyway, he just wants Tav to see others if they want. It really doesnt need to go further than that. Halsin doesnt actively seek out anyone else anyway unless Tav does it first (or Tav has already told Halsin no)

This way all they need to do is record a few lines, add a few flags, and thats it. It would be relatively quick and cheap to do it that way than to try to make a whole complicated convoluted conversation about Halsin being free to still be poly and seek out other relationships and Tav being ok with it.