So because of policy choices made by app developers, you have to manage multiple cloud solutions.

Or as the GP suggested, forego the cloud entirely. iCloud and Apple’s built in iOS backup is not a magic bullet unfortunately.

By one lone outlier who decides for “security” that the don’t want to support the platforms backup solution. That app purchase didn’t have to do anything besides store information locally in their sandbox

Does Signal allow the user to opt-in/opt-out into their policy? Or are they forcing this policy on their users?

No. They do not allow users to opt in