I think you don't want to know the real answer. It sounds like you want a phone app, but what you really have to do is flush your phone down the toilet and use a totally different approach. Also, there is absolutely no way to avoid difficult opsec. The communications technology is irrelevant since the greatest vulnerability in any security system is the people who use it. Do you think the private messenger software will free sessions with your therapist from spying? Guess again.

As the saying used to go, you're seeking a Star Trek solution to a Babylon 5 problem.