I use Markor Editor plus Syncthing. Markor is open-sourced and has a plethora of features, as you can read on their github page[1]; among these features, it includes file encryption.

Syncthing is really easy to set up and works like a charm. If you want accounts and user management, Markor also works with NextCloud.

[1] https://github.com/gsantner/markor