An alternative could be setting the DLC during the main story itself. I'm not really sure how it would work, and you could argue that it already makes an already bloated act 3 even more bloated, it's part of the reason why I wouldn't be opposed to it being split off into having an act 4 down the line. But at least then, you don't have to worry about the power level of certain companions/origins and how that would affect the balance of the DLC. And it'd also give Karlach a way to have her story wrapped up during the events of the main game itself like with the other origins, though I still have my own reservations about the idea having to pay for DLC to actually get a full conclusion to a main companion's questline, especially with how it's still one of the most barebones quest in the game.
I guess this is also just me being a little bit selfish, but it'd also be nice to not have to steer Wyll to being the Blade of Avernus if I'm not romancing Karlach, i.e. a Durge character wanting to live a quiet life with Gale. The Blade of Avernus ending just doesn't really feel as satisfying as him succeeding his father and potentially creating long-lasting good changes for the city of Baldur's Gate.
It could be something in act 2. Maybe zariel sends in an invasion to get mizora back from her capture and views her capture as the ultimate slight to her power. Damon could hear about this forge from people escaping the hells in the chaos of the invasion. I doubt it but it would solve the power level of post act 3 stuff.
