Aaaand I finally found a solution which I like and I want to share it with everyone. So basically now I am using "borderless keep res" mode in MTA.
In the app Gamma Panel (link above) I set brightness profiles. One is "High" for MTA with a bit higher brightness and hotkey Num8, and one is normal, default, hotkey Num5.
Of course it would be pain in the ass to click hotkeys everytime to change brightness, so there is a solution for this also!
Install "AutoIt!", create a simple script and run it:
While 1
WinWaitActive("[CLASS:Grand theft auto San Andreas]")
Send("{NUMPAD8 down}")
Send("{NUMPAD8 up}")
WinWaitNotActive("[CLASS:Grand theft auto San Andreas]")
Send("{NUMPAD5 down}")
Send("{NUMPAD5 up}")
Wend
Now for me it works perfectly.
I am personally happy with this tricky solution. Good if someone will use it too.
P.S. never used AutoIt before, but hopefully did a correct script. It works then its correct I guess (CPU usage by script 0%)