HDR on Mac mini with LG 27UL600

I am trying to display HDR content from my 2020 Intel Mac mini onto my LG 27UL600 but just keep hitting a brick wall. I can get certain clips to play on YouTube, for example this one https://www.youtube.com/watch?v=74SZXCQb44s, however I am not always getting smooth gradients and the blacks are all horrible and blocky.

I have set the monitor to HDMI Ultra HD Deep Color. I am using a Belden HDMI 2.1 cable. I have tried setting the Mac to 1080 60p HiDPI, 108060p, 1080 30p HiDPI, 1080 30p. I have turned HDR on and off in system settings. In terms of nits the display appears to be getting up to just over 400, which is what I would expect and looks great, but the colour gradients and blocky blacks just look awful.

I have tried everything I can think of and I am stuck. Can anyone offer some advice or I have just made a very expensive mistake believing that Apple and LG can actual support this?