Coincidentally, I have been researching really private messaging applications for days and my short search and filtering of information led me to two apparently good options.

1. Threema, from what I saw, is the most striking and focused, all its advertising, being used by the Swiss exercise among other political actors, looks robust. and it is open source, and it is paid. approx $6.
2. Briar is another application that I found with solid features, open source, does not require a phone number, without a central server, the information travels encrypted and has functions to be used without internet access. and it's free to download. is in F -droid. The latter is what I am implementing as the first phase to communicate with my wife and colleagues with whom I talk about cryptocurrencies and other topics and who are on the same path that we are all seeking to learn how to be sovereign individuals. I hope my opinion helps you. thanks for sharing