The best times in Maine, from my limited experiences are later than earlier in the "Summer" season. It might be georgous, but it's more likely to foggy and cold earlier in the season. If you're not in a hurry, and are prepared to let things happen as they might (rather than stay on a "schedule") my view would be to stick north. There is SO much to see up there, I'd wait to head south toward Cape Cod until later rather than earlier. My view is that you'd be happier staying north of Cape Cod canal until later.

Must stop places south would Nantucket, Martha's Vineyard, and Newport. They're not inexpensive to visit, but not stopping there would seem to be a mistake. You could spend a day or two at each.

Until after the end of August, I think that LIS is pretty hot and not very windy -- especially the farther toward NYC that you get.

Then I'm not sure that I would spend a lot of time in Eastern LIS unless you hot and steamy.

My view is the best time for the Chesapeake Bay after 1 September through the middle of October.