Hi,
Do you consider using jetson_multimedia_api? If yes, you may convert GREY to YUV444 and to encode into lossless h264 stream by running 01_video_encode. Please refer to this post:
H.264 lossless encoding - #2 by DaneLLL
For YUV420, we can only encode with qp=0 to get lowest loss. Your command looks fine. We suggest use nvv4l2h264enc since omx plugins are deprecated.