Brightness on External Monitor
I wanted to write a response to this question:
https://discussions.apple.com/thread/7239130
It is closed but none of the answers were satisfactory. I discovered that you can change the brightness on the external monitor by making it the primary display. In >System Preferences > Displays >Arrangement tab - drag the white bar at the top of your primary display over to the external monitor. You can then adjust the brightness on the external display using sliders or keyboard brightness keys. Once you have it as you like you can drag the white bar back to your primary display. No need for an app.
iMac 27″ 5K, macOS 11.5
