I’ve got an LG Ultrawide display on my Mac mini at home, and the keyboard brightness controls in macOS don’t allow me to adjust the brightness of the display.
This is usually not a big problem, except if I”m using my Mac late at night and want to drop the brightness down to reduce eye-strain.
In the past, I have used an app called Shades, however this app has long since been abandoned and was last updated back in 2013. It’s distributed as a .prefpane app and I’m not even sure that these apps work on modern versions of macOS.
In my searching I found a few replacements, and the one I’ve settled on is called MonitorControl. This app is quite minimal, and seems to work very well for my usage.
It allows me to easily drop the brightness of my LG display, and it can hook in to use the brightness keys on my keyboard, which makes it very convenient to use. One of the big things it does is that it supports multiple protocols to adjust brightness. It will use DDC for external displays (brightness, contrast, volume) and use the native Apple protocol for Apple and built-in displays. The app can also use Gamma table control for software dimming and shade control for AirPlay, Sidecar and Display Link devices and other virtual screens. The app advertises full compatibility with macOS Tahoe in the latest version, and seems to work well for me.
In the information for MonitorControl on GitHub, the author also mentions another app, BetterDisplay Pro which appears to do many of the same things, and a whole lot more, however despite being hosted on GitHub, BetterDisplay Pro is a paid app with basic features for free and the Pro features unlocked with purchase (and a 14-day trial for the Pro feature set).