Advice with SMBIOS/GPU Mojave setup in Clover

I'm working on upgrading my 10.13.6 build to 10.14.6, and running into a small snag with properly setting up my iGPU/dGPU and making sure I'm on the right SMBIOS hardware to take advantage of what I want to do.

Specs:

• Gigabyte Z270-HD3 Mobo (on latest FW)
• 16GB RAM
• i5-7400 (Kaby Lake)
• Migrated from GTX 1060 3GB to RX 580 8GB
• Clover bootloader
• Dual monitor setup, both attached to GPU
• Windows on separate drive

Kexts:

• WEG
• Lilu
• AppleALC
• IntelMausi
• VirtualSMC
• HFSPlus
• USBInjectAll

Drivers:

• ApfsDriverLoader
• AptioMemoryFix
• EmuVariableUefi
• HFSPlus
• NvmExpressDxe (my Windows drive is an nvme)

Previous SMBIOS - iMac18,2, with UHD 630 platofrm-id established by vanilla guide (00001259), framebuffer, stolenmem, etc. iGPU disabled. Current SMBIOS - iMac18,3, switched to 03001259 platform-ID as a reaction to re-reading the guide since it's been over a year (I am already planning on a conversion to OpenCore when I upgrade to Catalina)

The install went smoothly, but I got the choppy graphics after several reboots, couldn't open jpegs in Preview, issues that seemed to be common after some fumbling around. I did some searching through here, Google, and the Quicklook guide on InsanelyMac, and what I found worked was setting my BIOS as such - Enable iGFX (it was set to auto), check that DVMT was set to 64MB Min/256MB Max, remove stolenmem due to min being able to be set in BIOS, and then set initial display to iGFX, and not the PCIe Card (Rx580). This has me now booted in with no issues, other than I can't access my BIOS unless I swap a cable over to the mobo HDMI so I can see what I am doing, like the Clover bootloader if I need to boot Windows..

I checked out Hackintool (that is way over my head still, trying to learn that over a half hour is not enough time), and it is showing that my platform-id is actually 00001259 and not 03001259, which I found to be odd. System report shows both the UHD 630 and RX 580 as expected.

Now, the question part: How can I get my display to have my POST screen back on the GPU, and still have everything work ok? Should I switch to just the iGPU platform-id of 00001259, disable the iGPU, and switch the SMBIOS over to iMac18,1/iMac18,2? What about the catch-all iMacPro1,1? This is the part of the research I've been torn on, as I've seen a fair share of both sides of the argument.

My requirements:

• I don't need Quicksync if the RX 580 encoding is available to use for full acceleration.
• My understanding is that the iGPU is still used on the iMac 18,x series for some of the macOS features in Mojave+. Whatever is best use/closer to a pure Mac experience is preferred. (working on the continuity stuff after this is solved)
• I do very, very light gaming. My focus is more on basic video/photo editing/encoding.

Advice is greatly appreciated!