Lunar

macOS lets your brightness keys drive the built-in screen and then gives up the moment you plug in a Dell. Lunar fixes that by talking to the monitor over DDC, the same channel the monitor's own buttons use, so the keys, the menu bar slider and Control Center all move the real backlight. Once that works the rest follows: brightness synced across three displays, a curve that follows ambient light, and a schedule that pulls everything down in the evening.

The $23 buys Lunar Pro outright. The site calls it a lifetime licence and there is no renewal or paid major version in sight. The free build is not a demo either — it does hardware control, but stops at 100 adjustments a day, which you will hit quickly if you sync several displays.

Install the free version first, because whether any of this works is decided by your hardware rather than the app. Monitors that ignore DDC, cheap USB-C hubs and some docks leave you on software gamma dimming, which fades the picture instead of the backlight and looks it. And if all you want is the brightness keys working on one monitor, MonitorControl is free, open source, and does that much.