Yes, you can use it fully offline.
To back it up I believe you’d just need to backup your .pass
and .gnupg
directories.
I haven’t used keepass, but the entry from the archwiki should give you a good idea of usage, and it also lists some helper apps: https://wiki.archlinux.org/title/Pass
Yep, I’ve got a stack of 5-10 year old optiplexes (optiplexi?) running proxmox.