I just learned that there are programs to control the brightness of external monitors just like you can adjust your laptop’s integrated display. On windows, the most well known one is monitorian (FOSS), on linux you can (on Gnome) even use shell-extensions to have a brightness slider just like you do for the integrated display.
I might be out of touch, but is this well known?
It’s called the DDC protocol by the way. Like someone else mentioned, Twinkle Tray is a great option for windows, as is ClickMonitorDDC if you don’t want to use windows store apps.