GrapheneOS I'd say is the most reasonably secure open source Android ROM you could utilize (most practical private mobileOS at that). There are some flaws to GrapheneOS and that's delayed vulnerability patches which can be disastrous if you're a high profiled individual. Naturally since...
Tox routed over Tor is my go to solution, but that may change soon. P2P networks still have their inherent vulnerabilities but with Tor layered over Tox you're reasonably secure.
I've read many great things about Session & the Oxen network itself. Only issue I can think of is that servers are still by managed by developers of the Oxen network. The Oxen network also lacks large adoption which is why I'm sticking away from it for now. Apparently they are implementing a...