I set up S3 Uploads and object storage as outlined here in the OP, but without a CDN.
For the DISCOURSE_S3_CDN_URL variable, I have this:
https://my-bucket-uploads.s3.dualstack.us-west-2.amazonaws.com
All seems fine, including backups, however, in the console this error shows up when a reply to a post is started:
The request url in the error is actually a string of two urls which seems like the cause?
https://mydiscourse.com/t/uploads-test-for-s3/79/https://my-bucket-uploads.s3.dualstack.us-west-2.amazonaws.com/assets/markdown-it-bundle-a7328b73d3e7b030770eab70f10bdb0af655b3d8fa929bc49f1ad04c4cdaa198.br.js