I have the same telescope, ZWO filter wheel and ASI1600MM camera. Couple of things I found and fixed:
1. My carbon fiber tube had a loose ring on the focuser end. It's a really poor design to hold the weight of the tube and cell with four tiny screws through the carbon fiber. So I use a little epoxy and fixed this connection. https://youtu.be/OCN5rT-sJd0
2. The stock focuser sucks. I found it was not collimated well and the rotator was just sloppy. Using a laser collimator I discovered the focuser axis did not align with the center of the cell. It was off my 4mm or so. I replaced the focuser with a Moonlite and I'm very pleased this this! Highly recommended. The alignment looks great now and the Moonlite is solid.
3. Even after the items above I still had poor looking stars in the corners although the corners are now pretty closer to the same. Now I started experimenting with all my field flatteners: a) William Optics 0.8x reducer/flattener. Not terrible but still not "acceptable" to me. b) Televue TRF-2008 0.8x reducer/flattener. Little bit better than the William Optics but not much. c) Stellarvue SFF6 flattener only. Still poor in the corners, almost looks like astigmatism. Also, not sure yet but I think I'm seeing some sort of reflection issue. So I'm going to stick with the TRF-2008 for now but am considering trying the Orion flattener you're using.
4. I then tested Astrophysics method of spacing the field flatteners. I did this for the William Optics and Stellarvue. They talk about adjusting by a few millimeters but what I found with my setup is I needed an additional 10mm spacing to get the focal points the same between center and the corners! This gives me the flattest field BUT it's still not perfect! https://astro-physic...d-flattener.pdf
So now I'm trying to figure out what I think is a reflection issue. I'll post separately on that. I'm also starting to suspect the ZWO filter wheel is causing some of my issue.