iirc syncthing is encrypted, which matters because it will pass your data through a relay if it can’t connect directly.

check out deltachat, it’s an email client with an interface like an instant messaging app

NixOS has the potential to do really well here. The Nix language has a rich enough type system to generate GUI forms for every field, and there are several projects being worked on that allow editing NixOS options from a GUI. They’re still very janky, but it’s definitely possible to get to a point where a layperson could operate them without breaking their system.