I had a go with average stacking in dds and it gave me a lot more noise, it was a no go.
i use automatic backgorund extractor in pix in sight, other programs have the same or similar thing ie photoshop ectt, this should help even out values on the pixels that are affected by light pollution,vignetting ectt. and give you a better starting point to further process
I did not exactly record my process on that image of yours unfortunately , but i used a methode off ,load up the image- background extraction - a large histogram stretch and clip out a bit of background (noise and lp) - scnr reduction -green - then another automatic background extraction - used the curves with a large saturation adjustment, balanced the rgb by eye, move onto the lum channel curves for brightness, then noise reduction with tvgdenoise , then a small blurring with atrous wavelet transformation - a very small crop for the bad corners -save as jpeg 90%.
your flats looks problematic, that t shirt doesn't seem to be dispersing light evenly. i can see that the flats are good elsewhere- the darker corners and center, just the t shirt diffraction , i recommend using layers of opaque perspex spaced from each other . make sure the focus is unchanged from your astro session as well.
in dds where you load your images at the bottom screen, there are columns that list , file,type,score,dx,dy,angle,date/time ectt. along that row if you scroll all the way to the right there should be fwhm,sky background , those two are the most useful to manually analyze the group and toss out bad subs as well as score column too, get rid of a (if you have any) selection of high fwhm subs and high sky background, lower the better and get rid off low scores subs,higher the better for the score column
scopes :gso/bintel f4 12"trust tube, bresser messier ar127s /skywatcher 10'' dob,meade 12'' f10 lx200 sct
cameras : asi 1600mm-c/asi1600mm-c,asi120mc,prostar lp guidecam, nikkon d60, sony a7,asi 290 mm
mounts : eq6 pro/eq8/ (upcoming : mesu 200 v2)
filters : 2'' astronomik lp/badder lrgb h-a,sII,oIII,h-b,Baader Solar Continuum, Thousand Oaks Solar Filter.
extras : skywatcher f4 aplanatic cc, Baader MPCC MKIII Coma Corrector,Orion Field Flattener,zwo 1.25''adc.starlight maxi 2" 9x filter wheel