BrightRight runs on a schedule using launchd and synchronizes your MacBook's display brightness to all other attached displays using AppleScript.
The default synchronization interval is 5 minutes and is configurable by changing the integer value associated with the StartInterval key in com.BrightRight.RunAtInterval.plist. The integer value is in seconds.
Clone this repo somewhere on your Mac, and run install.sh. You should be prompted by macOS to give assistive access permission to BrightRightRunner.sh. After selecting Allow, you will still need to browse to Security & Privacy > Accessibility in System Preferences, and click the checkbox next to BrightRightRunner.sh.
BrightRight installs per-user, and will continue to run every 5 minutes while your computer is awake. It will not interfere with sleep, and will run automatically when the computer wakes. The System Preferences icon will appear in your dock temporarily, while the sychonization occurs, and then disappear.
Run uninstall.sh.