A reminder that iCloud Photos is not end to end encrypted, and that both Apple and the US federal police (FBI et al) have warrantless access to the contents of iCloud, so you are creating a huge trove of data that could be misused against you by police at any point in the future should it be politically expedient to do so. Screenshots frequently contain all sorts of extremely sensitive information.
This may not be part of your threat model, but it should at least be known by people so they can evaluate the risk themselves.
Everyone you iMessage with will still be putting all of your conversations and attachments and iCloud message sync keys into non-e2ee backups from
their end, so turning this on won't accomplish much even if you know about it.
It doesn't matter as long as the person storing screenshots in iCloud turns it on.
> Everyone you iMessage with will still be putting all of your conversations and attachments and iCloud message sync keys into non-e2ee backups from their end
Weren't we talking about storing screenshots in iCloud photos?
But I still wouldn't trust Apple 100% : we know that they were among the companies silently cooperating with the NSA, and the potential for backdoors in their software isn't nil. (Whether you should consider this as a real threat depends on your circumstances of course.)
This may not be part of your threat model, but it should at least be known by people so they can evaluate the risk themselves.