Sorry to be the bearer of bad news, but the Mac mini's have never allowed brightness control using after-market external displays or after-market input devices.
You must use the displays on-board menu or find some type of third-party brightness control software that does not mess with your color profile or FCP work.
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AI Overview
macOS does not natively support direct software or keyboard brightness control for most third-party external monitors connected to a Mac mini.
How to Adjust Brightness on a Mac Mini
- Use the Monitor's Physical Controls: The standard way to change brightness on an external display connected to a Mac Mini is by using the physical buttons or the on-screen display (OSD) menu built into the monitor itself. [1]
- Install Third-Party Control Software: To adjust brightness using your keyboard or menu bar like a built-in Mac display, you can use free or paid third-party utilities:
- MonitorControl: A popular, free, open-source app that lets you control external monitor brightness and volume using standard keyboard keys or the menu bar via the DDC/CI protocol.
- Lunar: A feature-rich app offering hardware and software dimming, with a free manual mode and paid adaptive features.
- Software-Only Dimming Apps: Apps like QuickShade or Mac App Store screen filter tools apply an overlay or gamma adjustment if your monitor doesn't support hardware DDC control. [1, 2, 3, 4]