Display dimming in the dark environment. (low Brightness)

I totally agree with this. You also should add an option to disable this dimming functionality in the settings, or control its brightness somehow. Is this even documented anywhere? Seems like this is some "hidden" feature that has been implemented to Fenix 8.

For example during the winter when I'm using long sleeved shirts and the watch is under my sleeve and I get a message, it is first very dim and then after 2-3s it will get brighter when the watch gets more light. Really annoying.

I totally agree with this. You also should add an option to disable this dimming functionality.

It's under Focus Modes - Sleep, you can adjust the brightness there. 

it has light sensing and automatically adjusts the brightness based on ambient light. AFAIK this is not adjustable at this time to disable it. 

Everything is too dark. But it's the dimming that annoys me. I don't need it and at least the minimun brightness should be higher or configurable. 

Seems to me your unit is not working correctly. Max brightness on mine is comparable to phones.
You might want to take the steps I outlined if it doesnt work I would contact customer support.

Other people have noticed this as well. You can easily reproduce this by going into a completely dark room, turning your wrist to wake up the screen and then point flashlight at the watch and the screen will get a lot brighter. This is clearly a feature that garmin has implemented in fenix 8. Haven't tested if this is the same with AOD as well.

Ok it doesn't need to be a flashlight, just put the lights on and the same will happen.

My wife has Venu 3 and the minimum brightness for that is a lot brighter than in my fenix 8 when comparing side by side. Hopefully garmin will fix this asap. 

This can be improved a little by setting the general brightness of the watch to maximum, but it affects also brightness in other ambient light levels and then it's too high on the higher ambient light levels. So Garmin, please adjust the dimming so that it will match the previous watch models (I didn't notice this feature on those models at all, but clearly it has been there before).