If you trust that you can opt out of iCloud Photos to avoid server-side scanning, trusting that this on-device scanning only happens as part of the iCloud Photos upload process (with the only way it submits the reports being as metadata attached to the photo-upload, as far as I can tell) seems equivalent.
There's certainly a slippery-slope argument, where some future update might change that scanning behavior. But the system-as-currently-presented seems similarly trustable.
I bet it'd take a while. The initial sync for someone with a large library is big, but just turning on upload for new pictures is only a few megabytes a day. Depending on how many pictures you take, of course. And if you're caught, an anodyne "a bug in iCloud Photo sync was causing increased data usage" statement and note in the next iOS patch notes would have you covered.
And that's assuming they weren't actively hiding anything by e.g. splitting them up into chunks that could be slipped into legitimate traffic with Apple's servers.
