> E2E and open source, and which has synchronization
I think you've set your expectations too high. You should be handling sync and encryption yourself; if anyone asks you to use their server to store your stuff you might as well assume it's not private anymore.
Use Syncthing or Nextcloud for your sync solution, and then use Zim Wiki or Obsidian as your notes app. The paid "all-in-one" solutions are not worth it.
I think you've set your expectations too high. You should be handling sync and encryption yourself; if anyone asks you to use their server to store your stuff you might as well assume it's not private anymore.
Use Syncthing or Nextcloud for your sync solution, and then use Zim Wiki or Obsidian as your notes app. The paid "all-in-one" solutions are not worth it.