Controlling brightness and sound for second external display

I have two external LG UltraFine 4K's connected to a 27" iMac. When there was only one monitor, I could easily control the brightness for my built-in display by using F1/F2 and for the external monitor by using CtrlF1/Ctrl F2F2.

Question: In a 3 display setup, i.e. one built-in display and two external monitors (both LG UltraFine 4Ks), how do I easily control the brightness for second UltraFine?

Lunar is a third-party app that can adjust the native brightness of the Ultrafines based on where the cursor is.

The Hotkeys setting allows you to choose what displays get controlled when pressing the Brightness keys alone or with the Shift or Control modifier.

Disclaimer: I am the developer of Lunar