How to respond to journal suggestion we get a native-speaker editor when writing seems reasonable and one author is a native English speaker?
While one should avoid antagonizing editors where possible, I think it would be reasonable for you and your coauthors to respond with a matter-of-fact email that includes the following points:

(1) one of the authors, namely yourself, is a native English speaker;

(2) said author has re-read the revised manuscript, and did not find obvious defects in the grammar or idiom;

(3) it would be greatly appreciated if the editors or the referees could point out the exact places in the revised manuscript, where it is felt that further correction is needed to meet the desired standards.

If they are being picky in good faith, point (3) should not cause a problem. If they stall or ignore point (3), then at this point it might be worth asking other people in your field or related ones if they have had similar experiences with this journal or its editors.