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.
I have 4 6Tb HDDs which I got new from work: been running 24/7 for the last 6 years.
The 4Tb drives they replaced where running for the previous 10 years (bought new as well) and are still in a drawer, got replaced only for the opportunity to upgrade to 6tb.
If your refurbished last only 4 years, to be that is not a positive gain, you lost money. New drives should last 10 years in my personal (and debatable) experience. But I think those drives you bought will last more than 4 years.