ffmpeg and Nvenc HEVC - bad quality

To encode a live source I use:

ffmpeg -stats -f decklink -i “DeckLink Mini Recorder 4K@22” -r 25000/1000 -c:v nvenc_hevc -preset hq -profile:v high -level 5.1 -b:v 28000k -minrate 28000k -maxrate 28000k -bufsize 35835k -pix_fmt yuv420p -r 25000/1000 -s 3840x2160 testuhdhevc25p422.ts

But quality is bad, it looks like it has only 500kb bitrate, the detail is gone, mosquito noise,…
What parameters can I use to improve quality because I`m missing a detailed manual on the parameters possible to use.

I just hooked up a Blackmagic Camera via a 6G connection to the FFmpeg PC and with the following parameters, I get somewhat better quality, but still some very blurry spots and loss of detail on movements.
Currently I am using a GTX960 but would go for a GTX1080 so I can add more parameters to improve quality I hope. But where can I find the parameters possible for the NVENC_HEVC ?
And where can I change the parameters of the preset ?

ffmpeg -stats -f decklink -i “DeckLink Mini Recorder 4K@22” -r 25000/1000 -c:v nvenc_hevc -preset llhq -b:v 15M -minrate 15M -maxrate 15M -bufsize 25M -enableaq true -tier high -r 25000/1000 -s 3840x2160 test3.ts

I really would like to push the GTX960 and GTX1070 to the limit and see what it can do, however, I need someone who can tell me what parameters I can add to improve the quality…

you read this,maybe helpful:

and I saw you use ffmpeg,so I want to ask you a question about gpu h264 accellation encode.

qustion:Do you know how many simultaneous h264 encode sessions can run on Quadro M4000( 8 GB GDDR5) card or a geforce card;

I know from a test that my GTX960 can do 2

I just ordered the GTX1070 to be able to use the rclookahead feature and improve quality.

However, it is very hard to find all the parameters I can use with nvenc and nvenc_hevc on ffmpeg. I waste hours searching the web, mostly finding examples from users on forums… I wished there was more info the be found.

In the meantime I know that the GTX1070 can do 2 sessions as well.

Quality is not really better for live encoding then the GTX960. For live encoding it is still better to go for X265, libx265 on the cpu. The NvEncoder really needs b frames and yuv420 to compete.
So I’m back on the CPU live encoding to develop encoders, our customers need better HEVC live encoding then what is offered now