Hope for the Best; Plan for the Worst
Welcome aboard the RoadTrip America Forums!
I don't quite share your pessimism regarding the opening of the US-/Canada border. As cases and deaths continue to decline, even in the face of the delta variant, and vaccination rates continue to increase, I think both countries will want to open the border sooner rather than later. We simply rely on each other too much to allow those who refuse to be vaccinated no matter what to control everyone else's destiny. In that case, and depending on where, exactly, you're starting from in Michigan, your overnight stop to Quechee would be around Syracuse, perhaps somewhere at the northern tip of one of the Finger Lakes on the 'thumb', Oneida Lake if you're looking for scenery.
On the other hand, if the border indeed stays closed, and again depending on where you're starting from in Michigan, you could easily need two overnight stops. We simply can't be too much more precise on timing without knowing your starting point - 'Michigan' is simply too big and vague. However, if the border is closed, you will have to drop south of Lake Erie and therein lies the rub. If 'getting there' is important, then you'll have no choice but to use the Ohio Turnpike and New York State Thruway, both toll and heavily used by trucks, and neither terribly scenic. You could take a more scenic route, basically following US-20 rather than the Interstates but that would up your driving time into a third day on the 'Michigan' to Quechee leg. From Glenns Falls NY to Quechee the obvious route of choice is US-4 through the Green Mountains.
Quechee to Bar Harbor would be a relatively easy day's drive and I'd suggest that you stay inland rather than following the route suggested by most mapping routines to drop down to the coast near Portsmouth NH and take US-1/I-95 from there. That's a nice route, but again it's toll and subject to heavy truck/tourist traffic. Instead look at some 'back roads'. I'd probably lean towards US-4/NH-118/NH-25 through Plymouth to NH-175/US-3 around Squam Lake then NH-258 back onto NH-25 around Ossipee lake to Standish ME and ME-35 around Sebago Lake to North Windham and ME-115 to US-202 all the way to and through Augusta (again eschewing I-95). ME-3 would then take you to Belfast and US-1/ME-3 for the final run into Bar Harbor.
Again, while I can be fairly specific about the Quechee to Bar Harbor route (I lived in New Hampshire and Maine for many years) details on the 'Michigan' to Quechee route would need a more precise jumping off location.
AZBuck