HP ProDesk 400 G5 Mini boot loop after BIOS reset - had Proxmox working before

[Tyro Sageframe]

Got an HP ProDesk 400 G5 Desktop Mini (i5-9500T, 32GB RAM, 256GB NVMe). Had Proxmox running fine on it. Did a CMOS reset and now I can't get ANY Linux to boot - Proxmox, Debian, Ubuntu Server, nothing works. Everything hangs at ramdisk or black screens.

I've spent hours on this. Tried:

• Every Debian/Ubuntu installer variant
• PXE boot (actually got the installer to work but the installed system still won't boot)
• Debootstrap from SystemRescue (installs but won't boot)
• Every boot flag combination: nomodeset, i915.modeset=0, video=efifbff, module_blacklist, etc.
• BIOS settings: UEFI vs Legacy, VT-x/VT-d on/off, different video memory allocations, Fast Boot, Turbo boost, you name it

In pure UEFI mode it gets past the ramdisk screen then goes black. In Legacy mode it freezes at ramdisk. Same behavior with or without boot flags.

The crazy part is Proxmox was running perfectly before the BIOS reset. Same USB, same hardware. Something in the BIOS state changed that's breaking Linux boot.

Anyone run Linux on this model? What BIOS settings worked for you? I'm losing my mind here.