Interesting thought but Apple never gives choice on much (well except Safari UI now!), and I don't see them giving an explicit choice on this. If they really had for a goal to implement E2EE, I could maybe see this now that you are mentioning it.

However if I had to make a guess, they'll go server-side scanning only, and E2EE backups/Photos will never happen.

To me it's a pipe dream to believe they would offer E2EE, there's too much international pressure from governments around the world to keep iCloud backups open and going E2EE would trigger unpalatable retaliations.