Jesus christ I finally found the cause of my random crashes.
First of all I got rid of the D3DERR_DEVICELOST by displaying the game in borderless window mode.
Second the random STATUS_ACCESS_VIOLATION was caused by this mod: https://de.gta5-mods.com/misc/ambient-car-sounds#comments_tab
It replaces the ambient car sound by modded ones - some of them seem to be too much for the game to handle - I never tought of audio mods causing the crashes but since I got rid of it the game runs perfectly with +15 Add-On cars, +15 script mods, + HD texture mods.
Now I can die in peace - lol.