App has been under review for over 40 days - no response from support

After around 8 days of the app not being reviewed, I contacted support, as advised in the developer portal. I just received a reply saying "we're still reviewing your app". A few weeks later, I contacted support again, because there was no progress. No response. I've sent another message now, but what should I do if I get no response again?
Quite frustrating. I have helped publish 50+ apps for my clients in various app stores and am seriously reconsidering if I want to publish them in the Amazon store in the future if this is the experience I can expect.

Case: #8568295161

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