HP Z620 Bios blackout

[keypunch]

HP Z620
BIOS version 3.96
Motherboard - 158A (Ivy Bridge-E)
GPU: Intel Arc A770

I did a dumb.

Enabled Above 4G Decode, and everything worked as expected.
Flashed the modified UEFI - no problem.

Looked at GPU-Z, and noticed that "CSM disabled" still said NO. Didn't yet see comments that GPU-Z can't be relied on. BIOS doesn't specify "CSM", but was set to UEFI only. Thought maybe I was crazy and misunderstood the request, so I set it to Legacy. Now BIOS is black, hot keys to get into bios don't work, even the modified EFI USB stick that I have to enable/disable 4G Decode stays black. Windows boots fine.

I've successfully re-flashed the BIOS from Windows, still black.
I've tried setting the BIOS into crisis recovery mode, *EDIT - but the BIOS screen never comes back up - so I never know when to actually remove the USB stick. It stays in the 8 beep code reboot loop as expected.

I'm looking for ideas how to get my BIOS back!

EDIT - I booted into the modified EFI (modGRUBShell) and changed the 4G decode back to disabled while screen was completely black. After reboot, still black, GPU-Z now says 4G Decode disabled.

EDIT - I've also cleared CMOS several times, some with battery, and a few with clear CMOS button on Mb.