I wouldn’t backup the volumes directly. Better to use the mount points as you suggest then back up those mounted directories. If it’s a database that usually needs to have its records exported into a backup friendly format. Typically I will do a db dump from a cron job in the host system to summon a script inside a container which writes to a mounted dir which is the thing that I back up.

Jottacloud is pretty good. They have a Linux CLI too

Yeah that seems about right. Bunch of things that I wish were better but I am not going back. When I absolutely must there’s a VM for that.