edit: it breaks rule 3 as the formatting defined in the formatting syntax section cant be applied to instances. This is because an instance isnt a community. And because of that, we know that this community isnt for instances.
The best option is to never trust anyone. Depending on how the info is supposed to be used, you can setup a website that does those important things with the sensitive data (stored on the server) without exposing the sensitive data to the user.
No. Proton's sketchy business aside, I wouldn't put everything into one basket.
Bitwarden also supports a few email aliasing services natively using an API token.
Tauri could be useful here.