I know very little about riding a bicycle in Maine except what I experienced on the Northern Tier. Campgrounds along the NT in Maine seem to be mostly private, and pretty expensive in peak season (so you might want to go before Memorial Day or after Labor Day). There does seem to be a reasonable supply of Warm Showers hosts along the NT in Maine. I stayed with two. I also liked Whitney's Wilderness Cabins near Woolwich, which gets you a cabin with electricity but no running water for $25.
As you know, Maine has lots of sleepy roads, lots of trees, lots of rivers, lots of bridges, and lots of hills.
It seems a fair way from where you live to the Northern Tier, but depending on how much time you have, you could work your way up to Fryeburg, across to Bar Harbor on the NT, and then back home on a bus along the coast. Within Maine, the NT goes through Fryeburg, Lovell, Sweden, Bridgton, Naples, Webbs Mills, Shaker Village, Danville, Penleys Corner, Durham, Brunswick, Bath, Woolwich, Wiscasset, North Edgecomb, Newcastle, Damariscotta, Waldoboro, Warren, West Rockport, Rockport, Camden, Lincolnville Center, Belfast, Searsport, Stockton Springs, Verona, Bucksport, Orland, East Orland, Ellsworth, Trenton, Mount Desert, and Bar Harbor.