But you need to know that the resulting output file is not compliant with HDR10. (The video stream does not contain static metadata such as SMPTE ST 2086, MaxFALL, and MaxCLL, which is necessary for HDR10.) That means some ordinary HDR TV's don't recognize it as an HDR10 video.

As of now, NVENC encoders wrapped in ffmpeg cannot write the video stream metadata.

AFAIK, NVEncC (by rigaya) is the only free NVENC encoder that can write the video stream metadata. Not only static but dynamic metadata as well such as those needed for HDR10+, HLG, and even some Dolby Vision HDR videos.

But NVEncC is not available as a module for ffmpeg. It's only available as a CLI executable unless you're a user of Aviutil (a Japanese encoding software program). That's why ShanaEncoder cannot make use of NVEncC by nature.

There are some encoding software programs that can employ NVEncC as an encoder. StaxRip and Hybrid are typical of them. But since they are based on frameservers like AviSynth+ and VapourSynth, you need to know that there is obviously some learning curve.