Thanks again for your analysis. But I have some good news in that it seems Startools isn't the problem but AstroArt stacking is. I bought Astro Pixel Processor a while ago but never got around to using it properly but it has a good reputation for its stacking abilities. After learning how to use it I stacked the same data and calibration frames and it created a 32 bit fits file after almost an hour of stacking. This one works fine in Startools with no banding after Wipe and Autodev ROI.
I just remembered that I used the HCG (High Conversion Gain) mode of the ASI6200 which kicks in at gain 100, and is meant to preserve the dynamic range of unity gain while giving significantly lower read noise. This might be the reason for the stars seemingly not clipping. I also thought this HCG might be the reason for the banding but it seems that's not the case luckily.
Sorry for giving you the runaround, but at least you know the answer if someone else has the problem. Would you like me to upload the APP 32 bit stack so you can see what the differences are? It's 3.30am here now so I'm off to bed.
