I was researching something else when this thread showed up in Google. I don't know if you have since solved the issue, but I had something very similar with my AT115EDT that I recently resolved. In my case, the equipment is the AT115EDT, MoonLite focuser, ASI294MM with 1.25" AstroDon filters, the AT2FF field flattener (I have the reducer too but haven't used it in a while), and a Pegasus Falcon rotator. I saw the exact same ring as in your original post. Long story short, after meticulously checking each part of the imaging train the culprit was the focuser. Specifically, the adapter that threads into the 68mm drawtube on the MoonLite and adapts it down to 42mm threads. It's a flat black surface, but applying flocking material from ScopeStuff eliminated the large ring in the images. When I looked down the imaging train with the camera removed, I didn't initially see any reflections, but if I moved my eye slightly off-center, I could see it. The flocking on that adapter eliminated it both visually looking in from where the camera is as well as in the images. It was only present in the LRGB frames, the narrowband were fine. I'm guessing because I'm under very light polluted skies (Bortle 7/8).
Along similar lines, I had an earlier issue where I saw arcs in the corners, so it looked like a very large reflection that was clipping the edge of the sensor. That one turned out to be the filters. AstroDon 1.25" filters, and again, the inside of the mounting ring was black but looked a little shiny on the LRGB ones. I cut a thin piece of flocking material and carefully applied to the inside of the mounting ring. The next time I imaged, those artifacts in the corners were gone.
Dave