I just want to post a short update about my performance optimizations. As I wrote earlier, I can reduce my GPU load dramatically by using a smaller texture when creating the Depth Map. However, using a fixed light source that lightens the entire scene results in very rough shapes, because of the distance of the light source to some models and the small size of the Depth Map. This was the initial reason why I increased its size to 4096x4096 pixels.
In order to improve the quality of my shadows I had to either increase the size of the Depth Map which would have decreased my performance or to find a better placement of the light source. After some tests, I decided to attach the light source to a specific offset to my camera position. That means, moving the camera moves also my light source. This has so many advantages! For example, when I zoom faaaaar out, I do not see any shadows, because the light source is too far away. This is great, because the shadows would be so small they could be mistaken for visual artifacts. The more I zoom in the more shadows I see. This looks really great in-game! I can also keep my small Depth Map size which is great for performance! I could also replace some calculations in the shaders with constants, because the camera and light source also always in the same relation! Finally, I further improved my Scene Graph that is based on a quad-tree space partitioning algorithm to only consider visible objects for rendering. This improved my performance on planetary missions dramatically.
So far, I render each frame in default view in around 4 ms GPU time and 9 ms, when I zoom max. in. I am happy with that, but I will try to make further improvements based on your great feedback!