The only thing that you are missing is the ability to input a frequent traveler program that you want your rentals credited to.

I've had my own issues with Sixt over this and, unfortunately, YOU are the one that will have to monitor it. My experience has been that at each rental counter (ZRH, MUC), I have had to show the card for the program I want credited. They cannot store this info for some unexplainable reason. Their system is very primitive compared to Hertz, Avis, etc.