Clight VS xfce4-ddc-plugin

There is also Clight (https://github.com/FedeDP/Clight) if you want automatic backlight adjusting given ambient brightness. It supports external monitors through ddcutil C library and has many more features to offer.

Here are two suggestions from the Arch Wiki, try Clight or macbook-lighter. Clight looks to be continuously maintained, so I'd try that one first.

Clight can do that. See the config file.

Installed light, clight, clightd and acpilight

If you're using a laptop screen, there's Clight, which can turn your webcam into an ambient light sensor.

Clight is a lightweight C user daemon utility program that turns your webcam into a light sensor which it depends on to automatically adjust your screen’s display based on ambient light. Just like Redshift, Clight can manipulate the temperature of your screen and it derived inspiration from Calise.

Check out Clight. There's also a GUI for it.

The only downside to Clight? You need to build not only Clight and Clightd from source, but also 2 dependencies (libmodule and ddcutil), as well as the Clight-GUI if you want to use it. That is, unless you use Arch Linux / Manjaro (there are AUR packages available), NixOS, Exherbo or Void Linux, for which there are packages available.

Neat! I'm working on something very similar for Xfce, but invoking ddcutil natively: https://github.com/apsun/xfce4-ddc-plugin (very much WIP, currently only supports hotkeys and a single monitor)

I've found that shelling out to the ddcutil CLI directly tends to be "lossy" - as in, if invoked very quickly (i.e. via keyboard shortcuts), it will tend to race with itself and fail half the time. So far the best solution I've found is to run a daemon to queue and batch together multiple operations, which significantly improved reliability.