It is currently not fully possible to adapt the email templates that Project Server sends to users. It is not possible to customize the email subject, only the body. See also: https://community.sdl.com/developers-more/developers/language-developers/f/57/t/131864
I suggest implementing a simple way for admins to adapt the email templates including the subject. It would be valuable for users to e.g. also have the project's or phase's due date in the email notification so that you can more easily determine which task to work on first when you get a lot of notifications.