I just use nextcloud sync, there are other options avaliable however like syncthing or easysync.
I store 99% of things in the documents folder, then storage scope apps to that folder, e,g nextcloud is pointed to there only, and it basically hoovers up anything in there.
I would love to use Seedvault and a USB stick or dav, but unfortunately it doesn't work, hopefully GOS fix it soon, its been on the plans for ages now.