Given that a lot of Cloudron servers might be set up on VMs with limited space, having the option to store on S3/S3 compatible would make a lot of sense for this app.
The official module can be found here:
https://github.com/matrix-org/synapse-s3-storage-provider
I do not believe this will add too much to the package size, and the module is maintained by the upstream project, so this addition should keep the package clean.