For a command-line option, ddcctl ( https://github.com/kfix/ddcctl ) works nicely in my experience, with a Dell Ultrasharp connected via USB-C. I have a script hacked together to fetch the internal display's brightness once a minute and apply it, with a scaling factor, to the external, so that they're both effectively controlled by the laptop's ambient light sensor. It's really nice for working in a room with lots of…
I did recently write a script to read the room brightness value from a Hue Light Sensor and adjust my external monitor brightness, with some smoothing applied. It was triggered by being stuck at home like everyone else. Funny how similar thought processes can be sometimes. I’m now up to motorising my blinds so that the light sensor also controls the blinds to keep the room at the same light level through the day.
I miss the roof deck at work. This time of year, weather and meeting schedule permitting, I'd typically spend somewhere between four and sixteen hours a week up there, enjoying the heat and watching ospreys, gulls, and the occasional peregrine while I waited for builds or test runs or whatever.
I think like many folks lately, I seem to have developed a touch of agoraphobia this year. But I need to at least be able to see outside, or I start to forget I exist.