Hi there. I don’t really have any advice to share, I’m afraid. Five players and a GM has been my upper limit. It really does work better with fewer players.
I don’t care for Art Magic but I also don’t think it will help you in this regard. In my experience it is more intensive at the table and requires more rules mastery. Standard sorcery might seem more fiddily but players will only have a small handful of spells that they’ll get to know quite well. Once you learn the standard sorcery system, it is quite quick.
In my view, the whole of Burning Wheel is built to funnel characters into meaningful conflicts, by which I mean Fight, Range and Cover, and Duel of Wits. You miss out on a significant chunk of the game if you don’t use them. Much of the math around Artha and advancement is built around the assumption that groups use them regularly. Without them, Artha will bloat, particularly Fate points, and advancement will be slow because most players won’t get much more than 4 or 5 rolls each per session.
In my most recent two-year campaign, I aimed for two conflicts per session, which I think is the sweet spot. We hit it more often than not.