Remove defects from round object

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Hello everyone, I want to extract the inner circle like structure from this preprocessed image:
This is my best result so far (Green = Fitted Ellipse):
For the result above i used:
dilate -> findContours -> only keeping the second largest minAreaRect -> fitEllipse
For my needs the fitted ellipse is not precise enough. What i tryed already:

• approxPolyDP: Either way it fails to detect part of the inner "circle" or it includes a lot of the defects
• HoughCircles: The structure Iam looking for is just round but not a real circle, so it's way to unprecise
• convexHull: Deforms the "circle" way to much
• contourArea: Tryed to remove some of the blobs due to their small size but since lines have a small area aswell it didn't work

The perfect result (created by hand) the green part:

Any ideas how to get closer to this result?