Syncthing seems to be a good way to get real-time two-way sync so far, but I’ve had a few accidents while setting it up, overwriting latest changes. Probably my fault, but I’m glad that I have backups to other machines.
I tried owncloud years ago and I didn’t want something that heavyweight for file syncing. And I think it stores a copy of all it’s days in a database too. I just want to sync files in-place only. I haven’t tried that or nextcloud in a long time.