Flirting with pretty princesses can be damned frustrating at processing time unfortunately Dunk. We were lucky to get so many fog free'ish nights but M31 is still below 25 degrees at AF most of the time which is a lot of atmosphere to deal with.
For galaxy processing this one is a bit like the neb problems with M42. Large dynamic range, but also lots of stars to add to the headaches. You've got so many great suggestions there. All I could add would be to consider playing with:
- consider collecting a bit more blue data in the field to compensate for the low altitude (think of how orange the moon gets when down low). You've managed to get the blue star clouds anyway with whatever you did...
- initial Masked Stretch to help keep the stars down
- Many of the intensity transforms in Pixinsight on faint DSOs are best used with star masks as you've noted - things like median transforms, LAHE, unsharp mask etc.
- Later you can invert the mask and work on the stars a little - e.g. colour balance, deconvolution if so inclined, or morphological transform to erode them down a little. This can certainly make the galaxy pop, but also risks looking "fake" - there has been a representative spread of personal opinions already in your thread here whether star minimisation is good or not
- DBE: results can vary a lot depending on how samples are selected - for broad gradients across a field with a diffuse DSO in the middle less points can be better. Rather than auto-generating sample points, try putting points around the outskirts of the field but avoid going too near the galaxy. If you can see any outline of the galaxy in the correction frame you need to try something different. As others have said, don't be afraid to do a few passes to get it right. First effort might only have 6-10 sample points around the outskirts.
- Colour balance - you need decent flat frames if shooting RGB otherwise its so easy to skew the colour balance between the bright centre and faint outer regions using DBE etc. Linear Fit seems to help even out separate RGB frames (not sure what camera you were using - ignore if OSC)
Still think you've great a great image there. Lots more to celebrate than worry about
it's a little more colourful, a little more detailed and the stars are a little sharper, even though it's probably impossible to tell with the file size limit and compression 
