NixOS has transformed my use of Proxmox. I configure, build, and deploy everything from my nix workstation. I don't need to use the PVE gui at all. Proxmox is just a target, and I've abstracted things enough to where I can deploy the same machines to libvirt on a local machine too. Why would I need to let my agent into my PVE box? I haven't looked at incus, but if I wanted to run the full stack declaratively, nixos and LLMs are so powerful now that I would probably just say to run libvirt and ZFS on nixos natively.
I'm a big LXD and now Incus fan. But I went with NixOS rather than IncusOS for my latest build because I prefer the LTS linux kernel over the mainline kernel.
Uhh, whut? It provides a button-y interface, but you can do everything via config files and `pct` on the command line if you prefer.