guys, OP here, I think I got it to work. The missing link was the requirement to enable the Nvidia App overlay. This is not enabled by default so I had to go do that.
It's still a little wonky and I can't adjust HDR sliders in-game for some reason. But I do really like the way it looks. Note I'm using RTX HDR and not the RTX Dynamic Vibrance (which gives it that bright wonky colors).
Highly recommend!