I have no idea how feasible it would be, but a level 12 cap seems fine to me. I hit it not long after reaching the city because I wrapped up as much as possible before moving on all the time. However progression didn�t really stop because of all the high end gear I bought and looted in act 3.
If we�d be able to go higher, I might have just given everyone a couple of levels of fighter for proficiencies and action surge instead of keeping them all pure classes.