[Garry74]

Did you turn off HDR in the O/S?
Why would you use onboard GPU audio instead of nVidia audio?  You're just asking for problems.  nVidia offers multiple ports to send audio/video assuming your AVR is outdated and you need to separate audio and video.
1080p vs 2160p is obviously a difference.  Ease of use is only as complicated as you make it.  Start with getting a working model using just Windows Explorer first.  Add in extra things like KODI after.  If you can't set it up in Explorer, no amount of button pushing in KODI is going to help you.

I was using the onboard audio as my receiver cannot handle hdmi 2. So you are saying I can use the DVI on the Nvidia card to send the HD audio to the receiver even though the receiver is only hdmi 1.4?

I did switch off HDR in the O/S. So if I play a 1080p movie first it is displayed correctly. However, when I then play a HDR movie, Kodi just switches the HDR back on. If I then play a 1080p movie again, the resolution is 1080p but with HDR.

I also have to set the colour to rec 709 so that the 1080p picture is not washed out. Which as I understand it is pointless for the HDR picture.

So to sum up:

1. I can play 1080p24 at full screen, but with HDR. No good.
   
2. I can play 2160p24 at full screen with HDR. Good.
   
3. I have to use Rec 709 to stop the 1080p signal being washed out.
   
4. I don't have HD audio, but I can use the DVI connection to send it to the receiver even though is HDMI 1.4.