The donut (or rather the 'halo') issue with stars seems actually not related to the combo of Ha and RGB or focus discrepancies - I can reproduce these halos on single RGB and Ha flies as well as the stacks. Somehow it’s on the raw image and reveals itself when it’s stretched a lot. A mild stretch seems fine but anything more extreme (as AutoDev does in SatrTools) then these halos reveal themselves. Where do they come from? The Skywatcher f7.5 ED80, the Astrodon filters, the ZWO ASI1600MM-Pro, or are these sort of natural artefacts (a glow)? I’m stumped?
Looking at past images with my DSLR they are also there to some degree so wonder if it is just the scope. The intensity of the central white dot is more pronounced in this ASI1600 image and the halos are more defined than in the noisier DSRL shots, so perhaps I'm just seeing these halos more clearly with the lower noise camera and the less noisy tighter Ha data?