For best quality you absolutely need a two-pass encoding, where in the first pass you collect "statistics" which are used to allocate bitrate in the second pass

For best quality you absolutely need a two-pass encoding, where in the first pass you collect "statistics" which are used to allocate bitrate in the second pass where you do the actual encoding. I don't think NVENC supports this workflow, which is a big reason it's not suitable for quality encoding.