I get the advantage of fully declarative systems such as Nix, but everything you do in proxmox via GUI can be done via CLI: https://pve.proxmox.com/pve-docs/#_command_line_interface
CLI is first class in proxmox, I use the qm command for managing vms all the time. The networking is also just a file in `/etc/network/interfaces` that I modify with vim as needed.
Doing things via the CLI and editing config files doesn't automatically give you reproducability or audit trail. Last time I check etckeeper doesn't work with proxmox either (due to the proxmox FUSE mount in /etc).
CLI is dandy but Terraform is even nicer. I run something like ten Proxmox clusters now around the world, and an AI agent can deploy a fully terraformed VM for me, tied into monitoring, asset mgmt, EDR, etc in a few minutes...