@fastfinge I haven't actually set this up before, but NFS seems like the way to go around this (network filesystem).
@modulux The thing that scares me about nfs is that it’s an ancient protocol, and authentication and encryption feel like a bolted on mess. Especially because I haven’t set up Pam or whatever to share accounts between the systems. So I’m confused about what happens when nfs tries to sync permissions.
@fastfinge It's ancient, but there's been relatively newer versions. That said I'm only talking from second-hand knowledge since I never needed to set this up. Another option you might consider is sshfs.
@modulux @fastfinge NFS (even v3) with a VPN would be fine. I use it and works fine. I proposed Syncthing in the top post of this thread too.
There's a distinct lack of WAN-friendly shared filesystems. So I would suggest either VPN + traditional shared filesystem, syncthing, or something SSH-based (like the rclone you are already using, or the already suggested sshfs).