Hi David,
Nice work.
With modern cmos cameras you are better off taking flats slower rather than quicker. this is something to do with how they are processed in the camera. Perhaps check these videos out (think there are 3)
https://www.youtube.com/watch?v=Kkg441UBNpo
to me the images have the following areas where there could be the most improvement:
- application of flats. don't seem to be working correctly - i'd revisit them.
- it looks like your sub exposures may have some of them out of focus slightly or perhaps the optics were affected by dew. (might be worth going through your subs and culling out any softer ones and then restacking).
once those two are sorted you'll be laughing and it will be much easier to process the images.
as for the gradients on andromeda it looks like compression.
I hope that helps somewhat
edit:
i did a quick effort to remove the flat gradient and it looks like if you can follow the solutions in the video i linked it probably should sort it out. (the removed gradient still results with the concentric circles as per the video - which indicates 'flats-with-modern-cmos issues')