Hi Nigel. Thanks for uploading the data. I had a play with it - since we are getting nothing from home at the moment. My workflow seems to be significantly different to yours. I have copied what you did from your earlier thread and added my steps in yellow.
Here are the main processing steps I applied:
1) Dark/Bias/Flatfield correction, alignment and stacking in Pixinsight using Winsorized sigma clipping.
2) Linear fit of R, G and B using L as the reference.
I/We don't do Linear fit at all and I certainly wouldn't use Lum as the reference. When I used to do Linear Fit, I'd choose the 'darkest' of the colour frames as the reference.
3) Applied a histogram stretch to all frames (transferring the Screen Transfer Function settings to make these permanent)
This doesn't seem right. I would combine the RGB without stretching the individual stacks (using Channel Combination). I would try to get the colours right before stretching. I would do DBE to the RGB combination. I would then do a Background Neutralization and Color Calibration. I might do some SCNR at this stage. I would then do the stretch. With the RGB we tend to do a 'Masked Stretch' with the Background number set around 0.07 or 0.08. I am just looking for nice colour at this stage, the brightness will come from the Lum. At this point, I would check that the peaks of the histograms for the RG and B are aligned, and if not I would tweak the individual channels to achieve this. This gives me a more balanced colour at the end.
4) Performed Dynamic Background Extraction, subtracting the resulting background frames from images to remove some gradients and a few irritating flat field issues (some remain)
Do the DBE before stretching - see above
5) LRGB Combination performed
I did a DBE on the Lum then stretched it using Histogram Transformation. I did this in small stages. I always do repeated iterations - drives Gnomus mad - watching the Preview - trying to keep the star sizes from getting away from me. Once happy, I add the stretched Lum to the stretched RGB using LRGB Combination with only the Lum box ticked. I dragged the saturation slider down to 0.36 - this boosts the saturation. I ticked the Chrominanace Reduction box.
6) Increased saturation across the spectrum to bring out some colour.
With the combined LRGB, I check the histograms are still aligned and correct them again if necessary. I do some more SCNR - the Boosted Autostretch button allows you to see if there is still some green in there. I boosted saturation with Curves and with the Color Saturation tool - picking out the blue of the outer galaxy and the orange-yellow of the core.
7) Found that star sizes in the red image are larger than the blue and green. This was giving an excessive red tint to all stars. So I went back to the red frame, applied a star mask, followed by Morphological Transformation to reduce the red star sizes a little.
I didn't find a problem with this, since I didn't stretch the colours individually before combining them.
8) Re-did the LRGB combination + saturation increase. Found the red halo issue was helped, but not completely removed.
9) Applied a star mask and reduced saturation levels in the red channel for stars only. This removed the general red tint and brought out some colour in the stars.
10) Some adjustment of the R G and B curves and cutoffs in Histogram Transformation to try and get a colour balance in the galaxy which looked right (aesthetically).
11) Final tweak of overall RGB histogram to hide the worst of the background and achieve what looked like sensible brightness, contrast and colour balance.
I didn't do any of 8-11. I did an HDRMultiscale Transform to a copy of the image. Then I took the original and the HDR image into Photoshop and painted on the bits I liked. I also did some Unsharp Mask in Photoshop. I did find a problem with your Lum. There was a large ring-shaped artefact extending a quarter of the way up the frame. This is probably something to do with flats. To be honest I didn't look at the individual subs you posted, I just used the calibrated stacks. I 'fixed' the problem by placing the RGB only image over the LRGB one (in Photoshop again), changed the blending mode to Luminosity and painted over the artefact.
I attach a screenshot of the odd Luminance artefact. Then my finished image.