If I am encoding from UHD HDR to HD SDR x265 10bit, do I want YUV420P8 or YUV420P10?

[protectron]

So, with the default settings for both filters, ToneMap gives a very dull image and HDR10ToSDR gives a blown out image. Neither are accurate to a streaming source I am using as a reference. 
Any suggestions on best settings to use to get to the intended image?

Also, when working with a UHD BluRay remux, generating a preview takes a very long time (several minutes). It looks like it is loading the entire file every time it refreshes. Is there a way to keep it in a cache so it loads faster?