Repository Specifics
Whatever the choice you make regarding your repository, you should apply maximum care to its hardening.
Having an immutable filesystem is useless if you system can be wiped at a lower level.
Take care to restrict or simply cancel any possibility to access low-level configuration systems such as hardware management (IPMI etc) for your Linux or Windows servers, SAN, NAS, Object Storage, deduplication appliances etc, and apply relevant restrictions regarding access to administration interfaces.
Finally, any hardening concept that is applicable to the Veeam Backup and Replication Server should also be enforced at the repositories level.