I'm probably dumb, but why would that be proof?

I upload encrypted backups to a cloud service provider (AWS, Google Cloud). I go to another computer, download them, use a key/password to decrypt them.

Sure, I get it, you're typing in something that decrypts the data into their app. That's true of all apps including WhatsApp, etc... The only way this could really be secure is if you used a different app to the encryption that you wrote/audited such that the messaging app never has access to your password/private key. Otherwise, at some point, you're trusting their app to do what they claim.