the issue could start with a lot of concurrent users and a lot of small changes to file, due to SMB not working at FS lvl but at file level/block of data.
IF hetzner would offer NFS, that could be a solution or better even if there would be support for iscsi.
To know the backup scope, you can sum up all the apps listed there + email data + box data. Then you would have to multiple that by the number of backups which are stored (eg. once every day for 1 week or so) This would be the maximum of storage required using the tarball method. For rsync, it would be much less since it would do incremental backups.