I hadn't considered Syncthing. One could for example bake the syncthing protocol into an SSB-based app such that whenever a paired device comes online it automatically syncs data over so to the user things are seemingly centralised. The only risk I can see there is a case where Device A is turned off before Device B is turned on, so the sync wouldn't carry over. That's a small price to pay though I think, and something people could learn to work around.

It's funny, I do exactly what you describe, but with Joplin, though it never occurred to me to reach for Syncthing in this case. Thanks!