Hadnt seen that post with screen shots..
So what if (for testing purposes) you don’t set the card as primary in guest but enable it all the same as secondary. Keeping the monitor unplugged… using team viewer you can the look to see if the os is talking nicely to the gpu by setting the monitor of TV as display 2 and then use the identify display button in the desktop properties (resolution+++)..
Curious as to what stage you got to with the on board enabled. I use my onboard for my console and am able to pass through gpu without issue although board card noting other than hypervisor + Guest OS are the same..