For example you can use Simplex chat app that works both on mobile and desktop computers, and it has integrated private notes.
You can also use it for private chatting, audio/video calls, and you don't need to use any phone number or other identifier.
Database can be encrypted, and if you lose passphrase you lose your profile.
Another important thing is that Simplex chat accepts bitcoin, monero and other crypto donations:
https://github.com/simplex-chat/simplex-chat#help-us-with-donations