If your monitor supports DDC [1] you can control the brightness with MonitorControl [2]. It's fantastic. However it's basically impossible to figure out before buying if monitors do or don't support DDC. In general Dell Ultrasharps do, and I was surprised when the old Acer I'm on now did as well.

[1] https://en.wikipedia.org/wiki/Display_Data_Channel [2] https://github.com/MonitorControl/MonitorControl