I'm not sure about bicycling routes from Tokyo to Osaka. I can check for you.
You will have to avoid the Tokaido highway - and that will be difficult because all the other roads will be a complicated maze of small streets. Mountain ranges, poor city planning, and rivers limit the routes in Japan.
Your best bets are to find rivers that cover large portions of your route. Most Japanese rivers have an improved/paved bicycle/foot path that skirts the bank of the rivers for miles or even for the whole length of the river. Also, Look for major stretches of farmland. They have straight and level access roads for farmers that often go for many miles - out of traffic too. These roads link small towns and villages.
I also wanted to ride from Tokyo to Osaka, but my Japanese friends suggested touring around Tokyo, then taking a train to the next destination and ride the area there - do this all along the way to Osaka. Actually, based on what I have experienced, this is probably a good idea.
As for bike shops, they are all around Japan - in every town. You don't have to worry about that. HOWEVER, most bike shops are only set up for the drones of similar single-speed commuter bikes. If you have a high-end bike with special components that need work or replacement, you could lose some time looking for a specialty shop.
Camping is very limited in Japan. I suggest that you look for "minshuku" inns or Ryokan Inns. These are usually old houses that have been converted into something like a bed-and-breakfast. The prices are about $30 to $40 per night and usually includes breakfast and sometimes dinner. I did this last year and it was a great way to go. Almost every town has a ryokan. When you roll into a town, ask the police for help. They will guide you to the best ryokan for bicycle tourists.
Most Japanese bookstores have wonderful detailed street maps. You can buy them along the way as the availability of regional maps seems to be local. Some of the big bookstores in Tokyo have maps of every reagion of Japan, but the smaller towns only have local maps.