I have used Brightness Slider in the past but now use Monitor Control:
See this thread:
Easily control external display brightness using MonitorControl
I have been using this app for about a week now. As with you, I stumbled upon it. I have been using Brightness Slider, that app has not been updated for five years. MonitorControl doesn't do any more than Brightness Slider did, but it's being supported so I will be switching. I have an LG...forums.macrumors.com
Available here:
GitHub - MonitorControl/MonitorControl: 🖥 Control your display's brightness & volume on your Mac as if it was a native Apple Display. Use Apple Keyboard keys or custom shortcuts. Shows the native macOS OSDs.
🖥 Control your display's brightness & volume on your Mac as if it was a native Apple Display. Use Apple Keyboard keys or custom shortcuts. Shows the native macOS OSDs. - MonitorControl/Moni...github.com
Lou
Does it work with the internal display?