Thanks for this post, I was encountering similar issues using Hetzner object storage (backup of the cloudron server was taking hours, for only 20GB of data), then I switched to Hetzner storage box with the recommendations @nebulon indicated. The backup was done in a matter of minutes
For some reason my initial trial failed, and when I reconfigured the backup site settings with a new storage box it worked (maybe because this time I enabled External Reachability in Hetzner).