Right. Well without seeing the actual EFI it's very hard to determine what is actually wrong with the build.
However I can provide some pointers:
1) If you are using OC 0.7.8 I'd suggest to also update Lilu, AppleALC, VirtualSMC & Whatevergreen to the latest versions. Include SMCProcessor and SMCSuperIO. Make sure the OC 0.7.8 Opencore.efi and Bootx64.efi files are also updated.
2) Make sure the Device Properties for the IGPU is this:
View attachment 545659
and AMD RX6600 device property is this:
View attachment 545660
3) Make sure the Kernel settings are these (I'm using UTBMap & USBToolbox here, but USBMap or USBPorts can work too):
View attachment 545661
4) Disable XHCIPortLimit and enable a custom USBMap or USBPorts kext (if you have one). If you don't have one use USBInjectAll but leave XHCIPortLimit off.
5) Use the following SSDTs in the ACPI:
View attachment 545658