Thanks for the tip about the drainage, I will mention that tomorrow.
Regarding "auf knirsch": Well, what can I say, there seem to be enough experts around here. We have spoken with several civil engineers, tile/floor layers, and the manufacturer, the procedure is by no means unusual. Even internet research shows that it is by no means unusual.
Anyway, I thought the thread was called "Show us your terraces" and not "we criticize everyone's projects." One shouldn't be surprised that hardly any photos are posted here anymore.