There’s surprisingly many spots in the big cities (cafes, restaurants, malls) and also in the Shinkansen. That said, it can be kind of a hit and miss, especially if you leave the bigger cities.

Pocket wifi is overpriced IMO. eSIM with data-only is super comfortable and pretty cheap per GB. Get them in a tech store like Bic Camera.

However it may not be possible to create a hotspot. Regular data sim cards are also available and cheap. Not sure if they can have the same restrictions, or if those restrictions would apply to your phone.

I had good luck with free wifi at coffee shops. The fastest wifi I had in Japan was at a Starbucks. I haven’t had great luck with the free wifi on trains at all.

I have heard Ubigo is a good option for esim cards. Make sure to enroll when you have reliable internet access already.

If you’re looking for a vendor for your portable wifi, IVideo gave us a router with excellent speeds on my last visit, and the return process was a simple prepaid envelope. Make sure to double-check the opening hours of the shop if you pick up your router locally.