What is the right course of action here?

Your answer was correct because the cyclist is turning left across your lane, which means you must yield and wait until the cyclist has completed the turn before proceeding. According to German traffic rules, vulnerable road users such as cyclists have the right of way if they are already signaling and starting a maneuver like turning, especially when you are going straight or turning in a conflicting direction. Taking priority or using the horn to warn the cyclist is incorrect because it could endanger them or cause confusion. Patience and yielding to cyclists in such situations is the safest and legally proper behavior.