> a reliable way to sync files that doesn't leave them readable to Apple or another party
It's worth noting that nothing in iOS or iCloud prevents apps from encrypting data using a separate user key and then storing in iCloud. (Essentially, using iCloud only as a reliable substrate for syncing files it cannot read.)
That popular apps do not do this is likely due to lack of demand as much as anything. I am sure there are some content creation apps that do provide this functionality, if one is willing to search hard enough.
It's worth noting that nothing in iOS or iCloud prevents apps from encrypting data using a separate user key and then storing in iCloud. (Essentially, using iCloud only as a reliable substrate for syncing files it cannot read.)
That popular apps do not do this is likely due to lack of demand as much as anything. I am sure there are some content creation apps that do provide this functionality, if one is willing to search hard enough.