The server is a VM with lots of available space, providing a NFS storage. The clients are Proxmox nodes. To be able to backup VMs from one cluster, all nodes of this cluster must be clients. The clients are connected to the server via a Wireguard tunnel, but not among each other.

New client

In order to add a new Proxmox node, execute these steps:

Run ./scripts/wg_newkeypair.sh. This will generate a new private/public keypair for this host.
Create a new host_vars file for this host. You will need to set a unique internal Wireguard address within the subnet 10.5.31.1/24. Also add the generated keys in there.
Re-run the playbook. You can limit it to the server and the newly added host. The other clients will not connect to this host.

You will need to be able to decrypt the vaultpw.gpg file in order to gain the password for the Ansible vault encryption.

Establish new NFS connection in Proxmox

In the Proxmox UI of the cluster you would like to connect to the NFS storage, do the following:

Add a new directory under the nfs_dirs vars of the NFS server, and run the playbook. The name should be able to identify the cluster.
Go to Datacenter > Storage
Add a new storage with the type NFS. Give it an ID, and provide the following info:
- Server: 10.5.31.1 (value of the server's wg_address vars)
- Export: the location on the NFS server you would like to write to. Select the nfs_dirs directory you just created.
- Content: only select VZDump backup file
- Select the Backup Retention as wished. Check the other clusters to see how it's done.

Leave the rest as default and press OK.

Now, you can either set up backup jobs with this storage, or backup individual VMs with this storage as destination. Note that in order to see the backups on this storage, you'll have to select it on the top right in the VM's backup tab.