On my test server I use Proxmox Backup Server in a virtual machine to backup my VMs to an SD card.
I can restore backups locally and it is faster than restoring over the network from my production Proxmox Backup Server.
I use Promox Backup Server because it provides de-duplication and incremental backups.
I wanted to periodically push my backups over the network to a NFS share in case something bad should happen to my SD card.
I tried to get nfs4 to work but I couldn’t so I had to use nfs3.
Here is a website that describes how to set up a datastore with Proxmox Backup Server using nfs3: