Windows – GPU passthrough works with UEFI firmware but not Windows iso

I am trying to set up a virtual machine with passthrough graphics. I am able to get the passthrough working for the UEFI shell, but not the official Windows installer. I can get the Windows installer to work, but only with emulated graphics

This boots the windows installer in a QEMU Window:

sudo qemu-system-x86_64 --enable-kvm \
-name TESTVM,process=TESTVM \
-cdrom /media/big-tank-8TB/OSISOS/Windows/WindowsOct2018.iso \
-smp 4 \
-cpu core2duo \
-m 4096 \
-vga qxl

This also boots the windows installer in a QEMU window (still no passthrough)

sudo qemu-system-x86_64 --enable-kvm \
-name TESTVM,process=TESTVM \
-cdrom /media/big-tank-8TB/OSISOS/Windows/WindowsOct2018.iso \
-smp 4 \
-cpu core2duo \
-m 4096 \
-device vfio-pci,host=43:00.0,multifunction=on \
-device vfio-pci,host=43:00.1 

But if I specify the paths to UEFI firmware, I get the Tiano slpash screen and then the UEFI shell both on the monitor attached to my passed-through video card and in a QEMU window.

sudo qemu-system-x86_64 --enable-kvm \
-name TESTVM,process=TESTVM \
-cdrom /media/big-tank-8TB/OSISOS/Windows/WindowsOct2018.iso \
-smp 4 \
-cpu core2duo \
-m 4096 \
-device vfio-pci,host=43:00.0,multifunction=on \
-device vfio-pci,host=43:00.1 \
-drive if=pflash,format=raw,readonly,file=/usr/share/OVMF/OVMF_CODE.fd \
-drive if=pflash,format=raw,file=/usr/share/OVMF/OVMF_VARS.fd 

Why is the passthrough working only in the absence of the UEFI files? Or, why is specifying the UEFI files preventing me from starting Windows?

Edit: Tried downloading a different version of Windows (April 2018 instead of the October one), same problem.

Edit: Tried purging and reinstalling OVMF, but no luck.

Edit: I can get to the boot manager by typing "exit" in the shell, but selecting the available DVD drive (and all other options) immediately falls back to the boot manager.

Edit: Ran this:

-name TESTVM,process=TESTVM \
-drive file=/media/big-tank-8TB/OSISOS/Windows/Win10_1803_English_x64.iso,index=1,media=cdrom  \
-drive file=/media/big-tank-8TB/OSISOS/Windows/virtio-win-0.1.160.iso,index=2,media=cdrom \
-smp 4 \
-cpu core2duo \
-m 4096 \
-device vfio-pci,host=43:00.0,multifunction=on \
-device vfio-pci,host=43:00.1 \
-drive if=pflash,format=raw,readonly,file=/usr/share/OVMF/OVMF_CODE.fd \
-drive if=pflash,format=raw,file=/usr/share/OVMF/OVMF_VARS.fd 

When I got too the uefi shell, I typed "exit" to get to the boot manager. In the boot manager, selecting the available DVD drive instantly fell back to the boot manager.

I then added another DVD drive by Boot Maintenance Manager > Boot Options > Add boot option, and then selected that in the Boot Manager menu.
.

This gave me a very brief "press any key to boot from CD". If I am fast enough, this boots into the Windows installer BUT ONLY IN THE QEMU WINDOW. The screen attached to the passed-through card was black with a simple cursor, as opposed to mirroring as with the UEFI/Boot Manager.

Edit: I am trying to pass through an NVIDIA GTX1070. Mobo is ASRock x399 Taichi, CPU is Threadripper 1950X. OS is Ubuntu Server with XFCE installed.

Edit: If I proceed with the installation, I still have Windows in the QEMU window and just a TianoCore splash screen on the passthrough card. If I go to the device manager, Windows sees the card, but it is stopped for some reason.

Edit: I tried using these instructions to get rid of the code 43, to no avail. In order to try this, I used virt-install instead of qemu-system, and when doing this there is no TianoCore splash screen. But still code 43 when I get into Windows.

Edit: used dmesg to check for memory reservation errors as described here. Found none.
Edit: Also from the above link, used ROM parser and confirmed the presence of a "type 3 (EFI)"