Hmmm. Why aren't there more options for Poly? If Shadowheart is okay with Poly, why just Halsin?
I considered Karlach as a second partner, in part because, well, she's going to either die or go to Avernus, neither of which is great. Let's bring her in.
In fact, if you've had wine with Shadowheart and then let Karlach have you, you can actually offer Poly. But Shadowheart will step aside and say Karlach deserves your full attention.