Nice one, Andy! There's no substitute for heaps of data on dim galaxies
Looks like you have a little misregistration between the channels? I don't know of any magic incantation that will give you perfect stars. It's a matter of getting everything you can right at capture and data prep time and then hiding any remaining warts later in processing
As Greg said, halos are pretty common. Careful focus and control of dynamic range will help but it's harder to deal with causes related to the optics.
Cheers,
Rick.