@john_tedesco I'm not sure if you're aware, but being on Keybase means you can also offer your PGP/GPG key to anyone who uses it. For example, for me, in the screenshot, it appears below the "5 devices" link.
PGP/GPG is the form of encryption ProtonMail uses, so it is well industry-tested.
Threema is not widely used because it costs money. But if you decide on buying it and want to test it with someone, let me know and I'll give you my Threema ID.