Comment by JasonFruit

Your point 1 especially speaks to me. Phone-based messaging in general isn't appropriate for the things I would most like to be kept private between me and a recipient, because those sorts of things can't be created on a phone. I've found PGP pretty good for making that happen, when I'm working with someone who a) uses PGP also and b) exercises some caution when using it. I haven't found an option that I can trust that will work for me.