I went ahead and took the step of deleting libvirt.img and recreating one vm. No go.
I then reboot and created a new vm from scratch (in this case a PopOS vm), everything went fine. Then I recreated a windows vm passing through the vdisk.img file and no usb/video devices. That was fine. Then I passed through USB devices; again, fine. Then I passed through my RTX3060, and it freezes the UI.
It's clearly something to do with the Nvidia GPU. I did delete the Nvidia driver and redownload per ich777's recommendation on page 91 here: