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

Super simple.

pass list existing passphrase pairs

pass insert $SERVICE add new passphrase pair, substituting $SERVICE for whatever resource you're saving a passphrase for

pass $SERVICE print stored passphrase for $SERVICE to stdout

You will need to configure gnupg key pair before using pass, if you haven't already.