I use both Seafile and Syncthing for various things.

I’ve managed to setup seafile ok behind traefik as a reverse proxy. Seafile has a basic web front end and allows you to integrate with Collabora for editing open document files. Its fine to just shut down the docker and copy the database I’ve found for backup. Or you can copy the files themselves, if you don’t care about the change history (which I don’t). I have as script to do this each night. Seafood isn’t too hefty and runs well on an RPi, even the collabora integration does.

Syncthing is a really good piece of software and I use this to share files from my RPi to my phone, but bare in mind by default its send&receive both sides so its not suitable for backups in that state.

Let me know if you want to know more.