Right now, there is a BTT function to change the brightness of an external display. However, if you have more than one external display there is no easy way to change the brightness of that third display, or more ideally, all displays at once.
Would it be possible to add "Brightness Up (All Displays)" and "Brightness Down (All Displays)" as well as small step variants? Then, regardless of the number of displays, it would just alter the brightness of all of them simultaneously.
Unfortunately it's complicated.
What BTT does for the predefined action "Brightness Up/Down External" is triggering the system shortcut ctrl+brightness up/down. I believe in theory that shortcut should change the brightness of all external displays, but there is a long standing macOS bug which makes this work only on some systems.
For the brightness slider on the Touch Bar I'm directly accessing private API to modify the brightness for all externals (while holding ctrl), however some display also don't react to that.