Here is some images from the current state. I still have a decent render time, which i thought would have been way worse at this state. Rendering 30,000 trees in 10 minutes due to instancing is not very bad I think. However, this is only in half the resolution I want my final render to end up in. The textures of the mountains are still the old one, and as you see, there is some stretching/resolution problems when the camera comes to close. I will probably re-texture them by using several tileable textures and mix and merge them together using layered shaders with hand painted alpha maps. This was the the way i planed to do first, but I had no idea this very simple layered shader existed and believed i had to do it by connecting together a shading network on my own to achieve the same result.
I can also mention that I didn’t managed to hold my deadline and broke my time plan. But now I got a new deadline, and I feel great relief and think I can manage this without sleepless nights of crunshing.