First of all: thank you very much for all the great tips and the links to the documentations.
I already was trying the "The Basic Drill" mentioned in one of the links. Yet it seems it does not help much as soon as they have spotted you once.
Really helpful I found the suggestion from MaxPower, gunso and maturin. The key seems to be hardcover and relocation. With such tactics I can do the mission.
Regarding modding: Thanks for those information as well, RogueTrooper - yet I do not want to make the AI weaker than they are originally planned (I think maybe have a much more shallow learning curve when doing so) - I'm more looking towards good concealment tactic than making the AI precision worse. Though I will definitely take a look at ASR AI.
The problem was more like why the AI can see me at high grass, when I don't have a chance to see them. When I turn down the Terrain Detail to very low there stay some obstacles which can really be used for cover / concealment. All the grass/bushes that come from Terrain Detail seem to only block my sight not the AI's is there an affirmative on that?