My brother and I go out east every year to visit family. And my wife is from port Elgin so I've traveled that direction too. From Barrie I would head up to orillia and then back road it over to ottawa through Bancroft (you can look it up on Google maps, you'll hit towns like gooderham, denbigh, try to get to calabogie and take 511 south, it's a motorcycle mecca).
After ottawa, you might as well slab it towards Montreal but be sure to take the 30 around the big city, then the 10 into the townships. You are looking at 3 1/2 to 4 hours of slab work, but it's pretty much unavoidable. Resist the temptation to backroad it through the townships, it is just too populated and slow to be enjoyable.
Take the 55 south for a couple of exits, to Ayers cliff. Then back road it to the border. We usually cross at Hereford, which is a quiet little border crossing. Then we cobble together a route using highways 26, 16 and 6 (easy to remember), which take you across the northern part of Maine to the new Brunswick border.
Now you're in the maritimes and it's up to you where you want to go. We often slab it across to moncton to meet our family commitments 🙂, but I prefer nova Scotia, it is more scenic and the roads are more fun. You might want to cross over the bay of fundy at St. John, by ferry.
We have taken so many different routes... coming back through gaspe is fantastic, as is the north shore of the st Lawrence. It's a wonderful area of the country to explore.