Thank you for the tip. Maybe I will sink a stone ring where the water comes down and fill it with gravel as a local splash guard and infiltration option. The balconies will probably need to be renovated anyway in the next 2 years, so I'll bring up the topic then. Below that, the boundary stones and the Cotoneaster can do their job. Maybe also let one root at the bottom of the boundary so that it holds the slope even better. In any case, thank you for your input.