Light pollution is one contributing factor as to why you don’t see the fainter details. Even if there were no city lights (as you are hoping for in the Alps), 6 seconds simply don’t suffice to catch those. I know, you can’t expose longer due to the star trails, but consider using a wider lens. The Samyang 16mm f/2 is really nice for astro on a crop. With it you can expose up to 18 seconds without needing a star tracker.
True, but the resulting images are not summed up, instead the median pixels were taken to prevent noise. Still the resulting image was only exposed for 6 seconds, and not enough photons from fainter stars made it to the sensor.
Would it yield a better result, if you summed up, let’s say, groups of five images and did a median on those six groups?