You don’t. Pass through to a VM on ProxMox works fine without integrated graphics and a single GPU. The graphics card simply drops the physical host console after the VM begins to boot, and then displays whatever the VM is outputting.
You're right, you can do passthrough with a single GPU, and there are multiple ways to do that. But note a huge caveat: you can't run an X server, whereas being able to simultaneously run desktop apps in the host and VM is a huge reason many run Windows in a VM rather than dual-boot into it.
