What struck me right from opening this image is how smooth it is. As having done a 21 hours image myself I know that data can get very smooth. In your case this image is buttery (if such a word exists and if not it does now
), that being the background has no noise whatsoever and yet retains very good resolution. Some of us like to have some noise to aid sharpness but here the image does not seem to need it.
My only criticism and you already have addressed this is the core of M42. Everything else is superb.
Good luck with the APOD submission. I submitted my Thor's Helmet and did not get looked at. You have been published before and should feature on the site again.