Thanks for all your help, Sylvia.
Unfortunately, once I got it to 0021, it just hung on the BIOS screen (still on HDMI only, no output on the DisplayPort). I've returned that unit and just got a new one today with the same issue. Having gone thru a total of 5 NUC boards and 2 different types of NUC, I'm no longer confident this can be my desktop replacement, so I'm having to look at other options so as not to waste the money already spent on compatible parts (and hopefully still achieve the same form factor).
I've already spent $21 so far in shipping to return the previous one, so I don't really want to spent $42, but I don't think Intel makes exceptions on return shipping? I will still have to open a case at this point, I'm still trying to see if I can get a refund from the reseller instead (since I should still be within 30 days).