Can we drop the luggage and then park in the parkhaus???

Might not be there when you come back. Better to keep it with you and walk back with it.

I guess you could if you don't care about your luggage

No. You can drop off a passenger with the luggage and park the car, but I would not leave the luggage unattended. If your suitcases to, it's really easy to get from the garage to the bus loading area. There are elevators.