I did this before learning about other tools! But you can download ddcctl from GitHub, then create an automator "quick action" with no input from any application. Then I created a flow of "Ask for text" (brightness), "run shell script" (/path/ddcctl -d 1 -b "$1/"

Once you have that saved you can go to your Mac keyboard settings and add this new automator app as a new shortcut!

Or, use an existing app :)

MonitorControl achieves the same thing but using the existing keyboard keys: https://github.com/MonitorControl/MonitorControl (recent discussion on HN: https://news.ycombinator.com/item?id=23785291)