I had the same problem forever. Apple's own instructions are very outdated. "MacOS 10.15.7" This was my problem. I had Mac Mini updated to Catalina, and it would throw similar errors to what you show. The solution was to install Big Sur Beta. I used the public beta build rather than the certificate/developer method as it was more direct. I installed to a new clean partition. Then downloaded and installed the beta of Xcode from Developer portal, and Configurator from the app store. The IPSW for DTK was obtained from https://developer.apple.com/download/universal/ and then after booting into DFU on the DTK (unplug, hold power, plug back in, connect via USB-C to port next to HDMI) I dragged the Big Sur image over and hit "Restore." It looked like it had froze up at the same spot as yours, but after 10-20 minutes came through. Note that the first time you plug it in Configurator will detect it as an unknown device and ask you to download updates for it, but there are none so this will fail. After completing the restore it detects as a Mac Mini and will again prompt to download updates, but fail again. However by that point the DTK should be back in factory restored mode and can be setup directly. Good luck, as I hadn't been able to apply a single update to mine in 3 months and the support and documentation seems painfully inadequate.