Good stuff on this thread, including some practicalities. Rail pass unlikely to be worth it. OTOH get data from your phone via Airalo (assuming your phone is unlocked and accepts eSims). If you have a phone that only takes physical sims, go to a camera store. Also use Google Translate, though in the bigger cities you won’t have as much trouble with the language barriers.
Hiroshima was great, better peace park than Nagasaki, very moving. Try okonomiyaki there for a foodie experience. Japanese pasta is also good in Tokyo and elsewhere.
Of the convenience stores, 7-11 is the best only because it’s where you can get cash with your ATM card, no problem.
The Suica card/app can be used to pay for transit within cities and some convenience store purchases as well. If you load it onto your smartphone, it’s really convenient.