I tried some of your suggestions and the solution with the view entity does work after all, but honestly I don't want to use a view entity, because I need to fade it out, move it accordingly to the rest of my interface etc. A view entity is more difficult to handle in comparison to panels.
It's affected by lighting, and the map is pixelated..
I'm sorry, but this does not make much sense to me. It's a very easy solution and it works. Simply assign a material that deactivates lighting and maybe assign a bigger texture to the entity to avoid a pixelated effect. You can fade it exactly as you do with panels (view_entity.alpha = panel_main.alpha).
