I wrote a guide for how I use restic to back up multiple hosts to B2, might be helpful to someone:
I’ve been using this setup to back up four Ubuntu machines and it seems to be working well. I use pass to securely store the B2 application key and RESTIC_PASSWORD on each machine, and I give my main machine access to every machine’s backups so I can do all the maintenance (restic forget, restic prune, restic check) from one machine.