listing ffmpeg commandline options

trac.ffmpeg.org/wiki/HWAccelIntro

You can see available presets including lossless

with

ffmpeg -h encoder=hevc_nvenc

List of the hwaccels available in the ffmpeg executable, 

ffmpeg -hwaccels

So, added
-preset lossless
to my commandline, and it worked. I think it was something like

ffmpeg -r 30 -start_number -003875 -f image2 -preset lossless -i "OpenSpace%06d.png" -c:v hevc_nvenc ../filename.mp4

Got the message
The selected preset is deprecated. Use p1 to p7
(but it is doing the encoding.)
There is also -tune lossless option. But I didn't try that.