To learn the program and what settings to use I have rendered a 10 seconds 4K clip with different Presets: Medium, Slow, Slower and Very Slow. The resulting file show no difference in size or bitrate. When comparing them in FFMetrics there is no significant difference either. Every Preset renders in 29 seconds.
With Medium preset the settings looks like this:
crf=17.000 noise_reduction=8 opencl=1 preset=medium rc=crf x264-params=chroma-qp-offset=-2:aq-mode=3:aq-strength=0.800:rc-lookahead=110:keyint=480:min-keyint=3:bframes=11:b-adapt=2:ref=3:deblock=0:direct=auto:no-weightb=0:me=umh:merange=32:subme=10:psy-rd=0.77:no-chroma-me=1:trellis=2:no-fast-pskip=1
