I have a similar question, so I was really happy to see this post! I have been looking at options for a 4-day walk on the Nakasendo in October 2026.
I first looked at Macs Adventure because they offer self-directed walking tours in Japan and we were very happy with them for a 10-day hike on the West Highland Way (Scotland) back in September 2024. However, I am a little concerned that they just subcontract their Japan hikes and I wonder how deep thier knowledge or customer base is for Japan. I just can't find a lot of commentary about their tours there. And the other concern I have is that while a self-guided tour was perfect for Scotland, a tour guide could make sense for rural Japan (based on what I'm reading). And for some reason Macs doesn't transport luggage between inns on its Japan hikes, although I believe it does that everywehre else. And that was a lifesaver on the West Highland Way! https://www.macsadventure.com/us/walking-tours/asia-walking-tours/japan/
That led me to look at Walk Japan. Although the owners are based in Hong Kong, they seem to have good reviews and more local depth. And they offer guided walks. Their youtube videos are pretty low energy and not very compelling, though, so that caused me to pause and wonder about other tour companies. https://walkjapan.com/
I've read great things about Mt. Sobek but not sure we want to commit 8 days to this (their shortest option). And its pricey. https://www.mtsobek.com/trips/asia/japan/
Anyway, I am so happy you posted this and hope to gain some insights. Thank you!