You couldn't use Hetzner's storage box either way because the backup system utilises hard links and it's not possible over NFS.
You could look at a Hetzner cloud VM with a block storage volume attached which would cost €450 a month alone. There's also Netcup with a similar setup which will be €98 per month for 8TB.
Another option is to look for a storage VPS where the provider will allow you to setup the OS/partition the disk so you have a separate backup partition.
Yes, it's possible to use multiple backup role servers, however I think it would require some management from your side ensuring your websites are balanced across the different backup servers.
You also have the option of S3 backups but these aren't incremental.