Mac Pro 2019 - will not start - stuck in endless booting loop

I've just bought a used Mac Pro 2019 in order to upgrade my Mac Pro 2013. I got the two machines setup to use Migration and went through the process. At the end of that, I got a message saying that something else (I can't remember the text) needed to be upgraded, so I agreed to that. Now the 2019 machine will not boot up. The Apple logo appears and the first milimetre or two of the progress bar appear, then it halts, waits, fills up the rest of the progress bar and reboots...endlessly.

I've tried CMD R to get into Recovery mode with no success

I've tried CMD OPT R to get into Recovery mode with no success

I've tried resetting the SMC using the unplug routine with no success

I've tried resetting the SMC using CRL OPT SFT with the power butten with no success.

What next?