could you add a panic mode : if the user enter a pin code (he can choose the pin) delta chat delete all messages stored locally and all messages stored on the email server.
this option would be desactivated and the user must activated it, if he want to use the option.
third party software will not erase delta chat messages on the server and the user wants everything in one software. otherwise why not encrypt your messages with third party software as well and delete delta chat ?
you have to know what you want: improve delta chat or tell users to find third party software…