I bought a monitor that has a "minimum" brightness that is still quite bright. How can I change the brightness of the video output of the Raspberry pi, so that the signal going to the monitor is not as bright? Normally on Linux this is done using xrandr however this command does not work on the Raspberry pi 4 running the 64-bit OS.
You will have to use the monitors brightness control.
Brightness can not be controlled from the Raspberry Pi at the moment.