I’d strongly recommend taking frequent (automated) full backups if not using config management. With config management, you can maybe get away with only backing up application data. The best way to do it is to use something like rsync/borg in combination with ZFS/Btrfs. The idea is to take a snapshot of the filesystem and then back up the snapshot, which handles any consistency problems.

For low load systems, just using rsync/Borg by itself may be enough. The advantage of Borg is incremental, deduplicated backups.

I personally use borgmatic (which is a wrapper over Borg, that is nicer to configure). It comes with a fairly decent starting config: https://torsion.org/borgmatic/docs/how-to/set-up-backups/