Okay found a great app for this.
Open key chain. It is open source and uses gpg encryption standards.
For predetermined passwords
1 Install and open app
2 Go to side menu. Hit encrypt/decrypt
3 Hit encrypt text
4 Change settings to “use password”
5 Enter password and the string of text to encrypt, and hit button to copy encrypted text to clipboard
To decrypt
1 copy encrypted text and open openkeychain
2 go to side menu, hit encrypt/decrypt
3 select read from clipboard. Enter in password
The app also supports signing with public and private keys etc. Their is also an app called pgp clipper that should integrate this more seamlessly with the clipboard, but I haven’t figured out how to use it yet.
It seems that running this setup on a secure hardened, degoogled, android, like GrapheneOS, should makes this Meshtastic use case pretty freaking end to end secure, without any “theatre”.