Backup to Server with mulltiple User Permissions

I have a Backup setup which I would like to do with urBackup. However, I cannot see how this is doable. Therefore, I thought I’d run it by you to see whether you have ideas of how to solve this wit urBackup.
Currently, the backup infrastructure runs the following way:
We have multiple backup Storage servers in the cloud. Clients have fixed quotas of storage assigned to them. Let’s say for example that Brian is allowed to use 1TB of Storage. He has his own SMB share, which he can fill with up to 1TB of data. Currently most of the backups are done with Veeam.

I would like to be able to get away from using Veeam. However I want to maintain the ability to assign quotas and be able to provide SMB shares.
urBackup doesn’t seem to like SMB shares very much due to the case sensitivity issue. Seeing the storage the Backups are stored on are ext4 this shouldn’t be a problem. Should it?
The big problem I have which I don’t know how to solve, is how to write each customer’s backup in a different repository and write it with different credentials. Is that at all possible to do with one Backup server? Or would I need a urBackup server for each customer?
I already have a local urBackup server for each customer to do Backups to a NAS.

Thanks in advance for your suggestions