Just log into a remote shell and execute emacs-org-mode from there. Like, remote shells are free 2022, no? I agree that if all the devices can't generalize to one source then it isn't as rewarding.
Cool idea! I do like to be able to work on it offline, e.g. on planes, though.
Have you tried using Syncthing? It's open source, free as in beer, and should fit your usecase well. I'm using it for my Obsidian notes, and it's pretty much frictionless.