I have/had a ProtonMail account, and whilst it was great, I believe it was only end-to-end encrypted when sending emails to other people using ProtonMail…? Or at least that was my understanding at the time.

The apps back then weren’t particularly polished, so I ended up migrating everything back to iCloud.

To be honest, I don’t seem to have any issues with iCloud and everything just works. But that’s the problem with Apple, and how they “get” you.