Update: DTK successfully backed-up and then upgraded via the App Store to macOS 11.0.1. I suppose the other problems here were related to unreliable betas.

Did you resolve this? I have Beta 1 on my DTK and want to upgrade it to the final release of Big Sur 11.0.1. Are the people having problems in the same boat as the original poster? What versions (installed and upgrade) are people struggling with?