One must understand that E2EE is used when you don't trust your service provider to handle your data. In other words, the adversary in your threat model is the service provider - and in this case, Apple. And what good is that encryption, if Apple obviously can do almost anything with your device?
They can remotely wipe apps. They can force-install apps and force updates. It is not too far-fetched to think that they can just remotely copy anything stored on your device to their servers. So, with an adversary that capable, I'm not sure encrypted backups provide a meaningful improvement to security and privacy.
In theory it adds a speed bump. Apple as the cloud service provider can respond to the legal order by saying they don't have the key. And then the police can ask for a booby trapped update for just your phone which may or may not happen. Or they can lobby the legislature for an encryption backdoor for all devices which will force them to show their hand in terms of "lawful intercept" capability.
If you want maximum security use an air gapped computer. But that won't let you send messages on the go.
You can, with some inconvenience, use optical diodes to transmit data from a trusted input device to an untrusted network device for transport over tor, and then push the received messages over a second diode to a display device that decrypts the messages, so that even if you receive an exploit/malware, there is no physical connection that allows unencrypted data to be exfiltrated.