> I've been thinking about solving the backup part by letting one of the devices perform btrfs snapshots on its storage.

I do zfs snapshots on my storage server (home desktop), which is in Syncthing's "receive only" mode. I use it to sync my camera roll and Whatsapp media files from my Android phone. I use pyznap for snapshot management.

> Setting up a cron job / timer unit that commits the synced folder to a git repo is also an alternative

Another option would be to backup the synced folder using a backup tool like borg, restic, duplicacy etc. They have options to prune the snapshots.