I got the 4K AND 5.1 working from the PC! It's a nightmare hack but it worked. I ended up plugging an Displayport to HDMI cord into the TV. Boom, 4K at 60Hz. There is another cable from the HDMI to the audio splitter. Boom 5.1.

So, after all that, I'm still not getting HDR from the PC but at this point I'm happyish. I get 4K when I need it, 1080p when I don't, and 5.1 all around.

It works well enough....