I’ve read the entire iOS security guide (seriously; version ~9/10), and while I don’t remember the specifics, I distinctly remember coming to the conclusion that I could not use most iCloud features as they did not provide the same privacy as on-devices features. I distinctly remember that your photos can be seen by Apple as that was the main thing I wanted to know.

Some things, like the keychain, are not accessible by Apple, and only on your enrolled devices.