I totally believe this works for you. But in your case, isn't NixOS just another declarative orchestration system like Kubernetes? Similarly I can just run a standalone nginx, caddy with acme, and a coredns pod in a bare minimum k8s cluster.

Personally, I think the complexity is on the same level.

It really isn't comparable. Sure, nixpkgs is huge, but the surface area for what you need to understand and work with is considerably smaller. They aren't even really in the same domain anyways. I was able to get very comfortable with Nix(OS) in a single weekend, but it took me months to get to a similar level with the K8s ecosystem.

NixOS has no "runtime" or controlplane to maintain.