You seem to think that your phone's pin is used directly as some sort of encryption key for cloud backups, but that's not how these systems work. Your pin or face or fingerprint unlocks your phone's secure element, and the secure element contains a randomly generated high entropy encryption key for your data.

This is why you can casually change your pin code in your phone's settings without it chewing through your battery and data plan reencrypting and reuploading gigabytes of cloud data.