iCloud Backup stalled on iPhone 12

iCloud Backup never ends. It says remaining time 1 minute, switches to "computing remaining time"but never progresses more than 5%

This happens in an iPhone 12. I've opened a support case 3 weeks ago. I have been sent 2 iOS updates, and still the same. I have 78 GB free in my 256 GB iPhone. I have 172 GB occupied in my 2TB iCloud+.

Once the backup is in progress, the info on next backup size is undetermined. It says "Loading".

I have cancelled it many times, deleted all pending copies, restarted the device,

[Re-Titled by Moderator]