What I'm looking to achieve are three identical proxmox host boxes. As soon as you finish the install you now have three snowflakes no matter how hard you try.

In the case of smartOS (which I've never used) it would seem like that is achieved in the design because the USB isn't changing. Reboot and you are back to a clean slate.

Isn't this how game arcades boot machines? They all netboot from a single image for the game you have selected? That is what it seems smartOS is doing but maybe I'm missing the point.

It doesn't look like it's achievable with vanilla Proxmox.

I think if you really-really want declaratively for host machines, you'd need to ditch Proxmox in favor of Incur on top of NixOS.

There is also https://github.com/SaumonNet/proxmox-nixos, but it's pretty new and therefore full of rough edges.