What is the right behaviour now?

Your choice to brake is correct because in situations where safety could be at risk—such as when another vehicle is overtaking or when conditions ahead might require reducing speed—slowing down helps maintain control and prevents possible collisions. In German traffic rules, braking calmly and early when necessary, instead of sudden swerving or maintaining speed regardless of circumstances, ensures safer interactions with other road users. This behavior supports defensive driving, allowing overtaking vehicles to pass safely and minimizing risks, especially on urban roads with mixed traffic like cars, pedestrians, and cyclists.