Ooo, even more reason to set-up a NextCloud instance now! Previously, it wasn't really practical to set-up B2 as external storage because you'd need to also set up a compat layer.
Just had to be sure to omit the B2 external storage folder from the backups on my Nextcloud server.
Now only if Virtualmin (YC ‘08) supported virtual server backups to S3-compatible B2 cloud storage… There’s an open ticket for this at https://www.virtualmin.com/node/65024