Restic or Borg. For restic I use the great Backrest web GUI.
I mounted an USB drive to one of my OpenWRT access points and backup on that one.
Rclone or fuse can mount/access Google Drive and can be used as back end for your backup choice.
Simplest backup ever: restic/Borg on a folder on the same PC. Not very recommendable, but indeed a good starting point.
Zfs/brtfs seems a complex solution for a simple problem. True is that once you start eating you get hungrier so maybe worthwhile.