> The only good implementation I know of personally is Obsidian Sync

Obsidian Sync gets around a major platform problem with Apple iOS devices, which is that they don't allow one app to change the data of another. You can use Syncthing for local E2EE sync, but it won't work on your mobile due to this. It works fantastic machine to machine. I'm paying for Obsidian Sync now just to get around that, but it shows how some of the platforms are made to prevent functionality. Ostensibly its for security, but I'd argue the benefit is mainly financial for app makers (and therefore their app store cut).