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

[Selur]

To clarify, does DGDecNV hardware (GPU) only apply to VS filters?

yes, DGDecNV is only the decoder

x265 should be purely software (CPU), correct?

yes

Going off topic from the original thread, but how do I make Hybrid use 100% CPU power? During this encode it's using ~50%.
Do I adjust --frame-threads and --pools from their defaults?

My guess is that atm. Your filtering simply isn't fast enough to allow the encoder to work faster.
So if you want to use more cpu power use more demanding encoding settings.
You can try changing pools&co, but I doubt it will speed up things.

Cu Selur