I'm still not seeing any photos or even links to photos. Re new flats, you probably will need them because stopping down does change the vignetting (drop off). It improves it in this case, to the point where I can usually get away without doing a flat. Note that I am usually also cropping the final photos somewhat as well because I manually dither and need to trim off the edges that don't overlap in all of the subexposures. So the effects of drop off in the corners is lessened.
Btw, those crops were from Astrotracer shots, which I do exclusively now when I'm shooting with lenses (as opposed to prime focus through a scope). Astrotracer almost automatically results in dithering and cropping with a lens like this because the target moves between subexposures and needs to be recentered fairly often. (Ideally between every shot but I'm lazy.
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