If the function is greyed out, then the green cable does not receive a signal from the handbrake, as soon as the signal is there, the function is selectable and you can determine the dimming yourself.

There are also brightness settings in the display settings. I left the basic brightness more or less at default but set the dimmer to +48. This way, even when the sun is shining, it shines directly on the unit. I also set the dimmer to time because the auto mode does not switch when I want it to. Unfortunately, the settings for the display brightness are somewhat hidden.

I also have the sporadic problem that it darkens. Probably because the light is always on today...

As far as I know, there are only 2 brightness levels: To change, press the 3rd button from the right, then a control strip opens at the bottom of the display. On the left you can switch to day/night.

With pretty much all Android devices, it is the case that the brightness can no longer be regulated independently in energy-saving mode. Have you possibly switched this on?