Dib2Qm

Messenger that works via e-mail server, end-to-end encrypted.

Text messages are not only end-to-end encrypted, they also get stored on your device in an encrypted way. Images, attachments etc. can be sent (encrypted, of course, but since they depend on third-party programs, they are stored unencrypted). Dib2Qm (aka Dibdib Messenger) makes use of your device's 'Download' folder.

It is possible to use the address of your device's 'Google account', but they insist on you explicitly allowing messenger apps: you would log in with your browser (!), go to 'Settings', 'Forwarding...IMAP', 'Enable IMAP', 'Save', then to 'Accounts', 'Other ... settings', '...Security', '2-Step...', 'Off', and 'Connected apps', 'Allow less secure', 'On'.

Strange? It is easier to use an extra e-mail account for Dib2Qm, but depending on your provider you might still have to allow IMAP services. Both, using the account's address and using a dedicated e-mail address for Dib2Qm, have their advantages and disadvantages ...

Just try it out! -- on a non-critical device (And if your e-mail provider blocks it, use a different provider.)

If you want to do a complete re-start, go to the device's settings, clear the app data, then open a file manager, clear the 'Download/dibdib' data, and then you can start the app a-fresh.