Yes, Japan is actually not terrible for vegetarians/vegans, especially these days where even Mos Burger has good plant-based options. But you need good language skills and local knowledge. Even most Japanese locals won't know how to help.

For onigiri, there are vegan varieties even at combini like kombu, daikon, or ume-shiso. They don't have a lot of protein though. At a dedicated onigiri place you can often find natto, bean, tofu, or egg onigiri, as well as inari.

A lot of vegan restaurants are just marked as "Japanese restaurant" on Google maps. Many of them don't market or present themselves as vegan. There are especially a lot in Kyoto, which has traditional specialties like yuba and shojin-ryori.

If for any reason you want protein bars, Japan has a decent amount of them in drugstores (although I think North America's are better).