You might be right, the gradient in the top right hand corner could be light pollution. Of all the calibration frames, I find flats are the most worthwhile. They're great for dust motes and also for removing gradients in the image. I also use dithering because it helps with noise in my DSLR. I dither every sub and find it helps enormously.
You mentioned
"posting the original stack, in case anyone’s willing to take a look". I had a crack at processing the image included in your original post, however, I will only upload it if you're happy for me to do so.
Re-processing other peoples images is generally frowned upon, unless the person specifically asks.
43 x 5min should definitely get you good results. You're stars are looking great, nice and round. The focus appears good too, as far as I can tell
Cheers,
Evan