Best to buy regional pass?

Hello:
Will be traveling in Japan for 15 days:
Fly into Tokyo for 3 days, travel to Kyoto for 4 days, Osaka for 1 day, Kumamo Kodo trek ( Nakasenda trail for 5 days, ending in Nachi-Katsuura), then back to Osaka for 3 days, and fly home from there. I see that not all the buses that provide transportation along the trail accept passes, but some do.
Can you please tell me if purchasing a pass would be worth it, and, if so, which pass do you suggest?

Thank you