Quicksy is a chat app with user experience similar to WhatsApp, Telegram or Signal (sign up with an SMS OTP and find other Quicksy users automatically). Prav App is similar to Quicksy from a user experience point of view, but difference is in how much control users have over the service. https://codeberg.org/prav/ has the source code. Users pay for Prav. Regular subscription income directly from users makes it sustainable.