So I finally figured out what caused the issue. The main problem was the heavy light pollution from which I was taking my pictures. It enhanced the vignetting effect by alot and made it almost irreversible even with flats. By traveling ~10km out of the city centre I managed to get WAY better results, still showing some vignetting tho. However, after using a new method I found online (Roger Clark's method of processing) I managed to almost entirely remove the remaining artifacts, donut shapes etc. Funny part: I didn't use any flats, darks or bias frames at all. All I did was applying the lens profile correction and adjusting the white balance in Lightroom, convert the DNG files into TIFF and then stack them in DSS. If anyone is having similar issues, try that method. It works wonders!
Funnily enough I'm currently performing an extensive review of Roger Clark's controversial method. Its strength is that you only need the lens profile and no flats, darks or bias. Lack of lens profile is also its weakness - if you're using a telescope with no profile.
More importantly though, the ACR raw converter uses a variable gamma response curve which leads to colour saturation in the faint data and any light pollution subtraction is performed in this non-linear space, which generally leads to a bluing of the scene. I can immediately see these effects in your final image - compare your second result with your first result which looks more or less correct (apart from the coloured rings of course). Also, take a look at examples of Roger's work - they invariably suffer from crazy oversaturation.
I'll be writing all this up in the near future with examples that demonstrate what's happening and I'll post a link here on Cloudy Nights.
Edited by sharkmelley, 14 February 2018 - 02:58 PM.