One correction. The road does go all around Maui and Big Island. Kauai and Oahu, no.
But if you go up to Kohala it deadends.
I just looked at a map, and you can go up the west coast to Kohala, then down the mountain road, so you don't have to completely double back like I thought.
I did not know they completed the road around east Maui. It used to be a badly unimproved road that was "off limits" to rental cars.
I don't know how current this map is, but it shows two sections around both east and west Maui, which are "off limits" to rental cars.