I have recently been researching to improve my ways of communicating with my close circle... and among several options in my opinion "Threema" is a very very good option, only it is paid (although if I am wrong it is a one-time payment ) and of the free options of all the ones I saw were Simplex, Signal, Session and Briar. My short research and comparisons led me to the conclusion that the best option could be "BRIAR" and it is the one I have been using, it is quite simple and complies with encryption and privacy standards. Take a look, maybe it can help you! DYOR PD - Download it from F-droid