Precision 5820 - Secure Boot enabled - Can't boot to USB drive to install Win 11

I've been attempting to do a clean install of Windows 11 via USB drive on a Precision 5820 desktop.

The device is supported for Win 11 and passes all Microsoft PC Health Checks except for "Enable Secure Boot".

With Dell's proprietary BIOS/UEFI(v.2.19.1), when you enable Secure Boot there's no options to boot to the USB diskette anymore, only Windows Boot Manager.  When rebooting, I press F12 to see the one-time boot menu, but the only option that is not greyed out is the Windows Boot Manager.

My other problem is that when my school districts IT department re-imaged my devices they used MBR instead of GPT for the volumes.  This I can convert but it doesn't matter if I still can't boot to my USB drive.

So, how does one go about doing a clean install of Windows 11 via USB drive on this device?

...don't get me started with network deployment of Win 11 since WDS is now deprecated, that's a different can of worms.