@tacore that's more of a question for the nextcloud forums..
This was surprising:
c25098f2-1535-49e4-a438-ee9232fc5619-image.png
Are you storing all of your files in S3 as primary storage?
Multibucket seems to be what you want, however it's not clear what your use case is, your understanding of primary storage or storage class.
The GUI has external storage you can add multiples of.
https://docs.nextcloud.com/server/latest/admin_manual/configuration_files/external_storage_configuration_gui.html