Went on vacation and came back at this with fresh eyes. Still having the issue though. I think this may be related to trying to use it in a multipart app, similar to the issue I had here.
That Heroku is spitting out 2021-09-13T15:48:03.790014+00:00 app[worker.1]: [2021-09-13 15:48:03,789: ERROR/MainProcess] Received unregistered task of type 'long_callback_61ab3ce7f aee53c1837008c09a44970689f42580'. in the logs indicates to me that the problem is the lack of presence of the callback. I’m not sure how to get it to import it though.