GPU Installation Issue - BIOS WHQL Setting Related

###Troubleshooting Help:

What is your parts list? [Consider formatting your parts list.]

PCPartPicker part list / Price breakdown by merchant

Type	Item	Price
CPU	Intel - Core i7-7700K 4.2 GHz Quad-Core Processor	Purchased For $285.00
CPU Cooler	Corsair - H100i v2 70.69 CFM Liquid CPU Cooler	Purchased For $85.00
Motherboard	MSI - Z270 SLI PLUS ATX LGA1151 Motherboard	Purchased For $105.00
Memory	Corsair - Vengeance LED 16 GB (2 x 8 GB) DDR4-3200 Memory	Purchased For $135.00
Storage	Samsung - 850 EVO-Series 250 GB 2.5" Solid State Drive	Purchased For $99.00
Storage	ADATA - Ultimate SU800 256 GB 2.5" Solid State Drive	Purchased For $65.00
Storage	ADATA - XPG SX8200 480 GB M.2-2280 Solid State Drive	Purchased For $95.00
Case	NZXT - S340 (Black) ATX Mid Tower Case	Purchased For $65.00
Power Supply	EVGA - SuperNOVA G3 650 W 80+ Gold Certified Fully-Modular ATX Power Supply	Purchased For $65.00
Operating System	Microsoft - Windows 10 Pro OEM 64-bit	Purchased For $0.00
Wireless Network Adapter	TP-Link - TL-WDN4800 PCI-Express x1 802.11a/b/g/n Wi-Fi Adapter	Purchased For $30.00
Case Fan	Corsair - ML140 Pro LED 97 CFM 140mm Fan	Purchased For $30.00
Case Fan	Corsair - SP120 RGB 3-Pack w/Controller 52 CFM 120mm Fans	Purchased For $63.00
Monitor	LG 43MU79-B	Purchased For $550.00
Keyboard	Logitech - G610 Wired Gaming Keyboard	Purchased For $60.00
Mouse	Logitech - G502 Proteus Spectrum Wired Optical Mouse	Purchased For $35.00
Other	Harman Kar​don HK-595​ 2.1	Purchased For $0.00
Prices include shipping, taxes, rebates, and discounts
Total	$1767.00
Generated by PCPartPicker 2018-12-05 20:12 EST-0500

Describe your problem. List any error messages and symptoms. Be descriptive.

Until ~2 months ago, I had a GTX 1080 (EVGA OG FTW model) installed, and had 3x1080p monitors. Sold the 1080 and ran integrated since (didn't make any modifications to the BIOS, just uninstalled the nvidia drivers).

Ended up selling the 1080p monitors a couple days ago and picked up the 43" LG listed above.

Since the 7700k can't do 4k60, I picked up a used RX470 (Gigabyte G1 Gaming) on ebay (I've purchased and sold a good number of GPUs on ebay, this is the first time I've had an issue).

BIOS w/ 1080 and IGD was configured as such:

OC'd
Various settings changed (like VT-x/d, boot order, etc) - all working as expected
Windows 7 support disabled
Windows 10 WHQL/UEFI mode enabled
MSI Fast Boot disabled
Fast Boot disabled
Secure Boot disabled

Installing the RX470 results in a blank screen boot loop or just a plain blank screen (can't even get to BIOS) in any BIOS configuration where Win 10 WHQL is enabled.

List anything you've done in attempt to diagnose or fix the problem.

The SX8200 is my normal boot device, and has been working as expected, both with gtx 1080 and igd, with the BIOS configured as above.

I also prepared a USB w/ Win 10 installation media for testing.

1. Attempting to boot to SSD w/ RX470 installed and Win10 WHQL enabled results in a blank screen boot loop or just a blank screen.
2. Attempting to boot to SSD w/ RX470 installed and Win10 WHQL disabled results in booting to BIOS as no boot device is found (since the windows installation expects whql/uefi mode, I'm assuming).
3. Attempting to boot to Win10 installation media w/ RX470 installed and Win10 WHQL enabled results in a blank screen boot loop or just a blank screen.
4. Attempting to boot to Win10 installation media w/ RX470 installed and Win10 WHQL disabled is successful - I get to the windows installation prompts
5. Attempting to boot to SSD w/ RX470 installed, but IGD-multi-mon enabled and cable plugged into mobo, w/ Win10 WHQL enabled results in a blank screen boot loop or just a blank screen.
6. Attempting to boot to Win10 installation media w/ RX470 installed, but IGD-multi-mon enabled and cable plugged into mobo, w/ Win10 WHQL disabled is successful.
7. Attempting to boot to SSD w/ RX470 removed and Win10 WHQL enabled is successful - I get to my desktop
8. I've tried some other configs, but the WHQL setting seems to isolate the issue
9. So
10. Many
11. CMOS
12. Clearings

TL;DR

GTX 1080 seemed to support WHQL/UEFI mode, whereas the RX470 appears not to. Do AMD cards have this issue? Card was purchased used, is it possible a modded GPU bios would cause these symptoms?

Side note: I intend to do basically 0 gaming on this machine anymore. Its pretty much just for work (software dev). However, I didn't want to spend slightly less money on a very basic GPU (possibly with limited ports) when RX4/5 series are going for $100-120 used (and I can get multiple DP/HDMI). The monitor supports PbP, so I use it as 4 'screens' during the workday, running 2 or 3 of the 'screens' from my desktop and 1 or 2 from a docked laptop.