What is the right course of action?

Your answer is correct because, according to German traffic rules, to avoid confusing drivers behind you when you slow down, you should wait until the overtaking vehicle has fully passed you before adjusting your speed or position. Slowing down prematurely or making gestures too early can mislead other drivers about your intentions, potentially causing dangerous situations. Therefore, it is best to maintain your speed and only respond once the overtaking vehicle has safely completed the maneuver. This ensures clear communication and smooth traffic flow.