Mac Pro 2010 Server (5,1) cannot update firmware to install High Sierra, plz help!

Hiya:

I bought a Mac Pro off eBay from a large Mac reseller a little over a week ago, and for the life of me I cannot figure out how to get this thing to update the firmware necessary to install High Sierra (10.13). If I follow the required steps during install of the firmware, I have to type in my password and the system will shut down. I turn the system on & hold down the power button until the LED blinks rapidly, I get a long beep, the apple chime, and the screen sits there black as can be for about 5 minutes, I hear the apple chime again... goes to login window, and.... rinse / repeat. FWIW, I've tried holding the power button down after turning on until the long beep, after the long beep, during the apple chime, after the apple chime and probably a thousand other ways. Nothing.

Here is the specs of the computer:

OS: 10.12.6 Sierra, originally had 10.11.x El Capitan

Boot ROOM: MP51.007F.B03

SMC Version (system & Processor tray): 1.39f11

CPU: 2x Quad Core 2.4GHz Xeon

GPU: ATI Radeon HD 5770 1GB

GPU Rom Revision: 113-C0160C-155

GPU EFI Driver Version: 01.00.436

HDD: Original Hitachi DK7SAA200 2 TB from July-2010

Ram: 10 GB Hynix brand 1066 MHz DDR3 ECC all RAM slots filled (6x 1GB, 2x 2GB)

Keyboard: Wired Aluminum Apple Keyboard (2007ish) connected to front USB2 port

Mouse: Logitech M705 Marathon Mouse via Logitech RF receiver in USB port on keyboard

Display: Asus ProArt PA246Q 24" connected via DVI Cable (MiniDP to DP cable doesn't work on Mac Pro,... but DOES work on my 2014 Mac Mini)

Internet via Ethernet directly to my router

I obviously don't know the history of this machine but it seems like the graphics card is a genuine ATI HD Radeon 5770, or at the very least not a PC variant (it has the two Mini DisplayPorts and DVI port on back). Oddly, in the "About this Mac" pic, it says the graphics card is ATY, Hoolock (?) 1024MB, and the card had a piece of tape on it with "312" written with marker on it. Possibly, the graphics card has been replaced?

That being said, I'm experiencing other odd behavior dealing with the GPU such as... I don't get the boot selector screen if I hold down the Option key on startup (just sits at black screen), and I can't get the Apple Diagnostics screen if I hold down the "D" key (just goes to login window). I've reset the NVRAM and SMC numerous times. I have no problems once Mac is booted up. I originally suspected that the GPU was hacked or flashed, but now I'm not so sure?

If the firmware is to be successful, it is my understanding that the screen will turn grey (boot screen) and the DVD drive will eject / retract. I never got the grey screen, no progress bar, just a black screen... as if no connection. I can boot into recovery mode holding Cmd+R (sometimes, I often have to reset the NVRAM and/or SMC). I cannot get to the internet recovery mode holding Opt+CMD+R.

I have tried so many different things I honestly forget everything I've tried! I will diligently oblige to any suggestions given regardless if I've tried it already or not, I desperately want to get this issue fixed!

Thanks so much!!