Thank you for that very extensive answer. Downloading the demo was something I haven´t thought about yet and it surely helped me understand how I am supposed to deal with reflections.
So the end result is as follows: Reflections use SSR when they don´t destroy my performance. SSR doesn´t do its job, when I look downwards. The only way how to deal my problem is a Scene Capture Cube then. Before I start: Are rough materials the better way to go?