[Startrek (Martin)]

Looks to me like typical Vignetting caused by your image train as most coma correctors introduces varying degrees of Vignetting into your image train. My TS Optics GPU does with my 10” and 8” f5 Newts. The Skywatcher f4 coma corrector is based on similar design principles as the TS GPU , just not as many elements so I’m confident that’s your issue
Correctly Calibrated Flats should eliminate it.
I use Huion A2 and A3 dimmable led tracing panels , taped some black cardboard over the panel and cut out a circle 5mm wider than the diameter of my Newt so the light covers the whole OTA. I take my Flats at night after each session. Slew the scope so OTA is pointing up to the sky ( obviously not tracking ) Grab a white T shirt stretch it over the end of the Newt using a big elastic band. Sit the led panel on top of the Newt, run my Flats Aid on APT and adjust illumination until I hit my target and ADU values. I take 40 flats and 60 Bias frames, my 2600 camera doesn’t need darks as it’s dark current is almost zero. My flats calibrate out fine with Bias frames, nice flat field , no vignetting and no dust donuts. Each camera and scope requires different calibration requirements.
It does take time to nail it !!

Cheers
Martin