If you are willing to handle the logistics of the bus ride, the Petit du Nord to Montreal would be nice and maybe the first choice as a bike path.
I haven't riden from Sherbrooke to QC, but my sense is that that is not as nice as the earlier section. So montreal to Chambly to Granby to Magog to Sherbrook would be really nice as another option. Turning that lower section into a loop, around Granby somehow, would elimate the bus ride.
We enjoyed Rivierre du Loup to QC, but that was only like 1.5 days of easy riding. We also did one day on Rt #5, if that is the Chemin de Roy, to Tres Rivers, and I didn't like that as much. It was mostly along roads, sometimes on the busy road, and then sometimes on the old road, but it seemed to not go through any cute towns and limited services. It was pouring rain that day, so it would be nicer in better weather. Any day in Montreal or QC is a great day, so adding them in makes sense.