Bert this is a great (almost incredible) image.
The image sharpness for a 300mm FL is fantastic
Combining narrowband and colour is most difficult, especially if your aim is to get a good colour dynamic range.
As I have said before you really need to get that lens out under a dark sky for a multiple night image run, I am sure your gradient removal work is a lot of your issue. If you were to do this I think you would be astounded at what that lens could reveal
I think it is well and truely DM awards entry material, colour aint everything...sole
Mike