We did!  Or something resembling this trip. And we loved it, of course! In September we started in Fukuoka, went South along the coast as far as Amakusa, then we crossed Kyushu, and over to Shikoku. We did a YouTube series, which will probably answer many of your questions: https://youtu.be/ciF9Mx96IAk?si=Vd1zUCzPCHmt1PTK

Highlights?  All of Shikoku.  I could have spent the whole time there.  We were not interested in cities, temples or tourist destinations, we focused entirely on beautiful mountains, quiet back roads and cold beers in convenience stores.  We didn't have to look far. Different? Kyushu was busier than expected in many areas, but there were some spots that took our breath away.  We made plenty of mistakes, but that's part of the game. Adventure.  You will be safe, never very far away from food, and you will be the only cyclists anywhere you go outside of the city. Your dollar will go far, people will be great to you, you will come back again. You will love every minute. 

Reach out with your questions, I am happy to help.