At first thank you for your answers. Lets split it into two different problems:
I think the problem with the Tiled approach would be that I also have a Projection Texture AND Shadowmap. Finally I'm not sure about the maximum Texture Array size. Next question would be, is it possible to render the shadowmap directly into the array on a specific slice or to I have to copy the single shadowmaps into the array?
As I mentioned before, the current approach to render the projection (by drawing a frustum mesh) isn't really an optimal solution, because it waste a lot of GPU power. Please have a look on the attached Images. The first one shows a current approach and the second would be perfect, but I don't know how.
I also thought about an another approach for texture projection, please let me know what you think:
In a computeshader just use the shadowmap to reconstruct a worldPos, Project it back to Screenspace to figure out GBuffer coords and finally do the shading. But I think the problem here will be the undersamping/interpolation, because of fixed size of the shadowmap.