Issue with GPU passthrough

I can’t move past the initial step in forwarding my GPU to a HVM using Neowutran’s guide (Guide)
I identified the IOMMU group containing hardware for my Nvidia GPU. The Nvidia drivers are the only ones in that group, but there is additional hardware for some USB functionality on my GPU that is in the same group. There aren’t any other Nvidia hardware components in any other group.
I inserted the line ‘GRUB_CMDLINE_LINUX="… rd.qubes.hide_pci=01:00.0,01:00.1,01:00.2,01:00.3 "’ into /etc/default/grub to hide them, then rebuilt the GRUB according to the guide. My GPU is hidden when that line is active in the GRUB, as the GPU puts out no video when I switch to it. However, the service handling the login and desktop GUI still tries to use the GPU for output.
Am I missing something important to do in knowing the specific IOMMU group the GPU is in? Otherwise, what might I have to change to get Qubes to stop trying to use the GPU to display itself?