Okay...I ran the problem down to the ZWO filters being the cause of the ring issue.
See this thread for results of my test ZWO filters against the Meade RGB filters toward the bottom of page 1.
https://www.cloudynights.com/topic/600525-flats-issues-with-zwo-filters-and-un-correctable-ring-artifacts/
I believe the problem is either in the coating of your filters or the curvature (I am leaning toward the coating).
Kaos