Tanja, your pictures are fantastic considering the amount of data you've used.
If you don't wish to guide, you can still improve your images.
You'll just need to keep adding more data. Try stacking 60 or 70 images instead of 16. The more photos you add, the more you can stretch the image to reveal more detail.
One bonus of taking short subs is that you will retain your star colour.
Flats are really important, as you already know.