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.

2 months later

I've used SeedVault with a CalyxOS phone to my Nextcloud serverfor quite a while, I haven't had to restore anything but it seems to run without any issues.

Unfortunately, I have not had the same experience with GOS. It always tells me it fails. Indeed the only way I can get it to work right now is if I save it locally, which I agree isn't that useful.
I haven't had any luck setting up DAVx5 with GOS, so that's not an option right now either. Sigh.

I just figured out how to have my tablet's NC app's AutoUpload service mirror my .SeedVault back ups, so it appears that this is a workable solution. Backups to local FS, NC AutoUpload of back up folder. Good thing I have lots of space for the local backup... for now anyway.