What is the right course of action now?

Staying in the middle lane is correct here because in Germany you must keep as far right as practicable, but you are allowed to remain in your current lane while an overtaking situation is still ongoing or not yet safely finished. After passing a slower vehicle, you first need to create a sufficient safety gap before returning to the right, and you must also check mirrors and consider faster traffic approaching from behind in the left lane.

The key rule of thumb is: use the left lane only for overtaking, and move back to the right as soon as it is safe and reasonable. “As soon as possible” does not mean “immediately after your front bumper passes the other vehicle,” but only when you can change lanes without cutting in, without forcing anyone behind you to brake, and while keeping a safe distance to the vehicle you passed.

So the correct sequence is: complete the overtake, stay in your lane briefly to build distance, keep observing the mirrors (especially for faster vehicles coming from behind), then signal right and return to the right lane when there is enough space. Moving left again would be unnecessary, and moving right too early could be dangerous or unlawful because it would reduce the safety gap to the truck you just overtook.