I went around with 2 wheels - but it was a bicycle this time so I can't really advise regarding motorbike riding specifically.. but i'll try.
During my ride along route 58 there were tonnes of local biker groups.
Regarding rentals in Okinawa I recall just 1 main shop RENTAL819 LOCATION｜RENTAL BIKE. It's a big chain that has outlets all across Japan and if I'm not wrong it's english friendly.
There is a youtuber by the name of Kansai Rider that does rentals as well around osaka Kansai Rider.
In terms of the road, route 58 is smack along the west coast most of the way once you leave the main city area. Going north, you'll have the beach on your left and some hills on the right. There's a couple of small waterfalls to explore and I happened to chance upon some dirt trails up a hill.
The region up north past nago city is sparsely populated and there isn't much traffic. This is where the bikers really opened their trottles.
I'd hope to be able to return someday to ride in Japan again for sure, bicycle or motorbike. The infrastructure is really good even in the remote areas.