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

[Selur]

You are using LWLibavSource in software decoding mode. (which for SD content usually is faster than hardware decoding, but for UHD and higher resolutions you hardware decoding is usually faster)
I would recommand to either enable the hardware decoding (Filtering->Vapoursynth->Misc->Source->Libav hardware decoding mode) or use DGDecNV. Unlike LWLibavSource creates a physical index file once upon loading the file and then reuses this, LWLibavSource creates the index anew each time.