I was able to fix it by deleting the problematic app screenshot set via fineTuneFork's instructions here: https://stackoverflow.com/questions/73313352/app-store-screenshots-screenshot-uploads-in-progress-error-appscreenshotse

Thanks, a feedback report for this issue already exists, I filed it on June 17: FB13938163 I also raised a DTS request the same day (citing that feedback report) as it’s quite a significant issue for me. I received this reply a few days later: Thank you for your interesting question. We'd like you to post your question to the Apple Developer Forums, and then reply to this email with the URL. Once we receive the URL to your forums post, one of our engineers will respond to your post directly in the forums. By responding to your question in the forums, the entire developer community will benefit from our discussion. Hence the original post here. I was hoping for just a little more but maybe the DTS response was sent by mistake. 😅