Download: here (macOS >= 10.14.4, signed and notarized)
This is a small menu bar app to mitigate the problem of the LG UltraFine not being able to automatically adjust brightness. It will poll the brightness of your builtin display and synchronize it with your LG monitors. So you can use your MBP's ambient light sensor for all your (UltraFine) displays! This also means manually adjusting the brightness of all your displays is easier, a single swipe over the brightness button on your Touch Bar will do.
The difference between this app and some existing apps is that this app uses a private framework of macOS to control the backlight of the LG UltraFine the same way the secondary slider of your Touch Bar will do. Other apps might virtually darken the display (with the backlight staying the same, this will reduce contrast) or use an older API to control backlight that won't be reflected by the system brightness sliders and may get overridden again. Also, this older API won't cause changes in the display gamma like the native system setting does.
Because it uses your laptops’ ambient light sensor, this won’t work in clamshell mode.
I only have one 27-inch LG UltraFine display, so I could do only limited testing. Let me know if you have issues.
Requires (and tested on) macOS 10.14.4, but could be built for earlier versions too.
You'll probably want to add the app to your Login Items.
The app polls the brightness pretty aggressively, which results in a small energy impact of around 0.3-0.5 according to Activity Monitor. However it will automatically stop the polling when no UltraFine displays are connected and because those monitors are also a power source, this effectively means it will never run when on battery power.
If you enter clamshell mode by closing your lid with the monitor attached, it will go completely bright for a second before restoring to the last synchronized brightness.