Most Japanese bus tours are highly regimented and completely inflexible. You'd do a lot better if you stepped up and paid for the services of a good guide, which would allow you to modify things on the fly if you so desired. I can't recommend anyone because I've never used a guide in Japan, not even on my very first trip.

I hope your visit in May doesn't include the first week of the month which is called "Golden Week." Things can get quite complex to arrange during that period. If it does include that period, better start figuring things out in detail immediately.

Few destinations reward advance preparation as much as Japan does. Do your homework, you'll really add huge value to your trip that way.