Easiest way to do this is to just do a copy of the videostream and insert a silent audio stream. I'm using the following to stream a 4k camera with 20fps to Youtube with 1-2% CPU usage:
ffmpeg -ar 44100 -ac 2 -f s16le -i /dev/zero -i "rtsp://xxx:
[email protected]:aaaa" -codec:v copy -codec:a...