Otherwise, you can let FFmpeg downmix to stereo automatically:
Stream #0:0: Video: h264 (High), yuv420p, 1920x1080, 23.976 fps, 5.50 Mb/s, 24 kb/s (bitrate) Stream #0:1: Audio: dts (DTS), 48 kHz, 5.1, 1.53 Mb/s young sheldon s06e15 libvpx
-c:a libopus -b:a 192k -ac 6
You should see: