How should I add redundancy to a small Hyper-V environment?

First decide whether the requirement is backup or high availability. Hyper-V Replica can maintain an asynchronous copy of the VMs and is useful for disaster recovery, but it is not the same as having services continuously available. For this setup, native redundancy is more appropriate: a second domain controller, DHCP failover, and DFS for the data. You still need independent backups, preferably on separate storage or a small backup appliance, because replication also replicates accidental deletions, corruption, and ransomware.

The minimum practical improvement is to use the spare server for a second domain controller VM and a second file-server VM, then configure the services to operate independently. Avoid putting every role on a domain controller if you can, and don’t treat two servers in the same rack as a complete disaster-recovery plan. A rack power, cooling, or environmental failure could still take both systems offline.

If possible, install Hyper-V on both physical servers and create a two-node failover cluster. Put separate VMs on the second host for the additional domain controller/DNS/DHCP services and the replicated file server. That gives you better maintenance and failover options than keeping a stale clone of the production machine. Make sure the design includes proper shared or replicated storage, since a cluster alone does not protect the VM disks.

Rather than copying the entire domain controller, configure the second host to run another domain controller VM. Active Directory and DNS are designed to work with multiple controllers, and DHCP can be configured for failover as well. For the file server, use DFS Namespace and DFS Replication so users connect through a namespace instead of directly to one machine. Print services are usually easier to restore from an exported configuration than to replicate continuously.