Thank you for your help so far! I was able to successfully install macOS after removing my GPU.
Having not seen your reply until now, I made a fresh EFI folder with OC-063-VISION-D.zip and used config-AMD-GPU.plist this time. I used your notes from the Edits/Updates section of your OP and copied my existing MLB, System Serial Number, and System UUID into the AMD GPU plist. I then tested it by replacing the EFI folder into my installation USB. After that was successful I replaced my Catalina SSD's EFI folder. No problems so far, and I'm seeing my GPU in About My Mac. I hope this was the right thing to do! If not, I have backups of my old EFI folder and system backup on a separate HD using CCC.
Otherwise, I'm excited to get to many of the post-installation tweaks and fix and bugs/issues that arise!