I stand corrected, the vast majority is not E2EE'd. Now there unfortunately is no information about the at-rest encryption "On server". Going by the iCloud encryption debate in china [1] and the alleged security modules used [2], they probably have the keys to "Photos" iCloud. Which is consistent with the Reuters claim that "backed-up [...] encrypted services remain available to Apple employees and authorities".

[1] https://www.nytimes.com/2021/05/17/technology/apple-china-ce...

[2] https://cpl.thalesgroup.com/encryption/hardware-security-mod...