Problems with M31 image quality

[fievelgp]

Hey everyone!   I'm pretty new to the hobby. Started very early this year with just a Canon Rebel T7 and a tripod. I got some practice in during the spring and summer just using the DSLR and the stock lenses along with a star tracker.   I since have acquired a Star Adventurer GTI , Askar 71f, and a Asiair mini along with guide scope and camera for guiding.  I finally got some clear nights recently and got roughly 10 hours on Andromeda from my Bortle 6/7 backyard.  

I definitely didn't do the greatest job processing this,  I know that lol. But  I can't seem to figure out what this red noise/gradient is. It seems to be in most of the images I have taken- although it doesn't seem to be in the Orion I took from the same spot ( I can post that too if it helps) .  I see it in some images on Astrobin with my camera, but definitely not all.     

I also seem to be missing something in my understanding of image scale/resolution/pixel size.  My image seems to be blotchy/pixelated on a desktop screen, but looks great from a smaller screen such as a phone (obviously). Again, many of the images I see on Astrobin with my camera look great at full resolution. 

I took this over 4 nights. I took flats each night. 4 of the 10 hours were during about a 3/4 moon.  ISO was set at 800 for 9 of the 10 hours ( I did 1 hour of testing at 400 during one of the really bright moon nights). Most of my subs were 120 seconds with 2.5 hours being at 90s and 1 hour being at 180s.  My camera orientation shifted over the 4 nights due to my rotator knob being loose ( I didnt realize it until the last 2 nights.) which made stacking difficult.  I stacked each of the 4 nights separately in Siril and then stacked the 4 masters together in APP as that did the best job of eliminating the stacking artifacts caused by the different camera orientations.   I didn't take darks,  and calibrated my flats by offsetting the bias when manually stacking in Siril . 

I would appreciate any feedback and thank you very much!