What is the right behaviour now?

Your answer is correct because in this situation, reducing your speed and allowing the car in front of you to merge are the proper actions according to German traffic rules. When approaching a lane closure, the speed limit often decreases, so you must slow down to comply with the new limit for safety.

Additionally, giving way to vehicles that are already in the continuing lane or attempting to merge ensures a smooth and safe "zipper merge" (Reißverschlussverfahren). This merging method requires drivers to use the closing lane fully up to the merge point and then alternate merging with traffic in the continuing lane. Accelerating would be dangerous and disrupt traffic flow.

So, slowing down and letting others merge first helps maintain safety and order on the road near lane closures.