After several modifications to my setup and a lot more practice I can finally do turns and come back to the dock. In the videos I use the 640 mm fuselage setting with a -0.75° spacer between back wing and fuselage. The maximum pumping distance at this point is limited by stamina and is approximately 100 meters. I am pretty sure that with a better pumping technique this can be improved a lot.
Up next is to get better in take-off (always land back foot first) and to increase the pumping distance. Stay tuned!