SilkenTofu

Google’s tried a few times. Allo, hangouts, and probably others.

I believe you can export joplin notebooks in several different widely compatible file types. So you have full portability, although perhaps not full interoperability (I use it in an encrypted form anyway so I couldn’t integrate any other apps).

Sync conflicts send the replaced version to a dedicated conflicts notebook, in my experience.

You don’t even need to trust the cloud storage as long as you have a sufficiently strong database password.