Yeah that's a big problem that I'm trying to research solutions for myself too. It was way better when I could tell people to just install Signal and it'd replace their SMS app but be secure when others use it, but unfortunately Signal dropped SMS. Currently I just have all the apps, but since Signal does contact discovery (like Whatsapp) I follow a Signal, Whatsapp, FB Messenger, RCS (via Google Messenger), then SMS pattern and stopping when I can contact someone. Obviously, this has the issue that all these apps are getting far more data than they need and I'd like to look into a multiplatform app that does e2e. From what I've researched so far, Matrix bridges (servers that connect your Matrix account to a third party messaging service) might be the answer.

I haven't tried it yet but there is a Matrix bridge that you can host if you are selfhosting a Matrix server (or use a commercial Matrix provider that already hosts it) that will allow you to connect to your Whatsapp friends without needing the Whatsapp app yourself that could be interesting for at least that use case https://docs.mau.fi/bridges/go/setup.html?bridge=whatsapp .