OK, I have had a look at the EFI, Hackintool files and DSDT you provided regarding your Comet Lake system. I would comment as follows:
SSDT's:
You are using a number of SSDT's that are set incorrectly for your system.
This includes the SSDT-i225V.aml table for your 2.5GB Ethernet port. I have edited the ACPI address for the Ethernet controller to match the one found in your DSDT.aml and Hackintool > pcidevices.dsl table, i.e. (_SB.PCI0.RP05.SL05), which is completely different to the ACPI address in your original SSDT-i225V.aml table (_SB.PCI0.RP02.D066).
I have used your System DSDT.aml and Corpnewt's SSDTTime python script to generate a number of new and replacement SSDT tables and companion config.plist patches.
View attachment 565066 New/revised SSDT tables
These are contained in the Results folder attached below.
I also created an SSDT-SBUS-MCHC.aml table for your system, as this is a helpful ACPI table when running a hack, as it helps macOS identify and work with common features found in a PC. See the link below to see what the SSDT does.
IGPU:
You appear to have disabled the Intel IGPU in your system bios, as it is not showing as being present in the Hackintool > pcidevices.txt or pcidevices.plist. I would recommend you enable the Intel IGPU in your bios, as your Comet Lake SMBIOS expects an IGPU to be helping the discrete GPU.
You have three entries under the IGPU's DeviceProperties in your config.plist. This is wrong when using an Empty/Headless framebuffer as you are doing. All you need it the AAPL,ig-platform-id entry. The other two can be removed as they do nothing for your system.
If you were using the iMacPro1,1 or MacPro7,1 SMBIOS you wouldn't need to have the IGPU enabled in the bios or any DeviceProperties entries for the IGPU in your config.plist.
Config.plist:
- I have edited your config.plist, to include the new/revised SSDT's and companion patches.
- I have cleaned up your config.plist to remove a number of unnecessary entries, so the config is easier to read and navigate.
- I have added a couple of DeviceProperties entries for your i225-V Ethernet and the Broadcom BCM4360 Wireless adapter, to ensure they are shown as built-in.
- I have removed the agdpmod=pikera boot argument, as you are currently using the RX580 dGPU.
Discrete GPU:
There is nothing in your OpenCore EFI that would mess with your discrete GPU, whether that be the RX580 or your RX6800 XT.
You don't have any SSDT's, kexts, patches, DeviceProperties entries or any other 'Fixes' in your setup that would cause any issues with your RX 6800 XT.
All your OC EFI contains dGPU wise is WhateverGreen.kext, which is expected and normally required for the IGPU and the dGPU to work in macOS.
EFI:
I would recommend you try the attached EFI on your system with the RX580 installed. Use it on a spare USB pen drive in the first instance, so you don't mess up your current booting OC setup.
You need to add your system Serial Number to the config.plist, but don't need to make any other changes.
Make sure this EFI works with the RX580 before trying it with the RX 6800 XT.
Config.plist- RX6800:
I have created and attached a second config.plist that is aimed solely at the RX 6800 XT card. This config should only be used if and when you are sure the config.plist in the main EFI works with your system.
It contains the DeviceProperties for the RX 6800 and the agdpmod=pikera boot argument, with the ACPI address for the RX6800XT taken from the Hackintool > pcidevices.txt but no other changes.
View attachment 565070
This config.plist will also require the Serial number to be added before it can be used.
ResetNvramEntry.efi:
Just remember that when you use the revised EFI and again when you try the second config.plist with the revised EFI, that you need to use the ResetNvramEntry.efi from the OC boot screen, before you boot macOS with either setup.
The ResetNvramEntry.efi will not be visible when you arrive on the OC boot screen. You need to press the Spacebar to unhide this driver and any others hidden by default by OC.
Hope this helps.
