Any luck on this yet? I'm having the exactly same problem with a NVIDIA Corporation GK110B [GeForce GTX TITAN Black] (rev a1) on my top slot with MAC bigsur. I did look it up and it looks compatible on the site mentioned above. I did dump the vbios and checked it on a separate VM (windows10) and finally got that working. So I was very hopeful that when I tried the MacOS it would work.
Any idea if the 6700xt would work with macinthebox or MacOS hackintosh? I'm trying to put together a compatible hardware list before i pull the trigger on parts.