Might the troubles you all are facing have anything to do with the BIOS version?
I haven’t tried any custom BIOSes, but I can confirm that I use VT-d with 4 8GB DIMMs with XMP on Z490 Vision D, and I don’t have any problems with the f5 BIOS.
But I had major problems with f20 and XMP on. USB disks were regularly being ejected during sleep. I downclocked the memory from 3200 to 2666, and that tended to help, but it wasn’t 100 percent foolproof.
I wonder if there’s a particular BIOS version you all should use?