Manually downgrading the aws gems in the container did get the uploads:migrate_to_s3 job to actually upload images so apparently the env variables wasn’t enough, as you pointed out.
However the process failed later at a different point:
FileStore::ToS3MigrationError: 2 posts are not remapped to new S3 upload URL. S3 migration failed for db 'default'. (FileStore::ToS3MigrationError)
That will be a separate troubleshooting adventure.