Have you looked at syncthing? I believe it ticks off all your boxes. I'm using it to sync my obsidian vault and Todo list between multiple android, Windows, and

Have you looked at syncthing? I believe it ticks off all your boxes. I'm using it to sync my obsidian vault and Todo list between multiple android, Windows, and Linux devices and it Just Works. I do have an always on server running it as well to both maintain a backup and to catch any notes devices powered down might miss, but it's not a requirement.