KDE connect and/or Synching/Syncthing-fork.

I don't think you're ready for self-hosting, but getting these installed and sorted out will help you move along that path, plus it will meet your needs in the mean time.

Bit, as others have suggested, get familiar with networking a bit.

You might want to get a raspberry pi or cheap SBC with a good amount of memory and disk space, and fuck around for a bit, trying some things.