Nope. This is all working fine with Mojave, so it's unlikely to be a hardware/bios thing. Further, if I have to disable IGPU, won't that affect my ability to generate H.264 H.265 videos? I have this machine specifically for working with FCPX.
Can you point me to the line in the verbose snapshot(s) that indicates the card error? I agree, just from observation, that you're probably correct, but I'd like to be able to recognize the line when I see it.
To my way of thinking, given that this is not a new install, and that I have an EFI folder that is working just fine under Mojave, the issue is with something else in EFI. It's not likely to be WhatEverGreen, or Lilu, since I'm running the exact same versions now, and there are no problems. So, I think that it's something else that is conflicting with them, either in the EFI folder, or in Catalina itself. If the latter, I'm SOL; if the former, perhaps I can (with your help) find it.
I'm now starting to think that I might want to mount the clone drive, boot from a Unibeast stick and I install a new Catalina onto the clone. See if that will get it to boot successfully. Although I've built and used the thumbdrive before, several times, I do not recall if I can actually choose the drive to install on... If so, then that may be the best approach, eh?
What do you think?