What is the right behaviour now?

You answered correctly by choosing to reduce your speed and drive as far to the right as possible. According to German traffic rules, when a faster vehicle approaches from behind on a rural road, you should make it easier for them to overtake safely. This means slowing down a bit to allow the overtaking maneuver and positioning your vehicle as close to the right edge of the lane as safely possible.

Signaling the overtaking vehicle is not necessary because the overtaking driver is responsible for judging when it is safe to pass. Your predictable behavior—steady speed reduction and right-side positioning—helps the overtaking driver anticipate your actions and execute the maneuver safely.