Initially I tried VGA — no dice. The machine said the console was on serial and no further output would be sent to VGA. I went into the boot options and set VGA to be primary, but didn’t have any luck there, either. It simply did the same thing as before. I tried from the EFI shell and that was about as useful as anything else I’d tried up until that point. In desperation I enabled the legacy user interface and disabled the serial port consoles. This finally got me going on VGA in text mode.
Looking back I probably could have gotten the same effect by completely disabling the serial consoles in one of the other interfaces as well, but it didn’t occur to me. Why would I need to do that? What’s the logic in defining primary and secondary console interfaces if the machine isn’t going to use them in that order?
The good news is that the machine is up and running now, and I hope not to have to bother with it again for a while.