For Misskey-powered instances, there's also Nijimiss.moe from the bigger ones I know about. It appears to either house or federate a lot of Japanese users too, from what I could observe. And dunno your Mbin instance, but on mine, I can follow Misskey.io and Nijimiss.moe users just fine, just taking a little while to populate the /all page (but surely happening since Japanese folks seem to interact between one another a lot), and at worse on federation to Mbin with some formatting issues for posts since Misskey feels a bit like its own thing.

But alas, on the flagship (?) instance, I know a guy that was able to create an account when he/she lived for a while in Japan, and when the person went to another country, access to the site remained.