I think the roads you described are as good as it gets for the Eastern half of the nation.
The blue ridge mountains heading even into PA offer some excellent roads thate are not to crowded. I think so of the best ways to do this is to figure on sites you may want to visit and taking the back roads to them.
The Matt Farah method of finding good roads: "Find mountains on Google Maps, look for squiggles". I don't have mountains in Michigan so I look for roads that follow rivers.