Similar questions pop up from time to time, see e.g.: Restructuring NixOS to work without systemd? e.g., with SysVinit
The answer is generally that NixOS is not optimized for minimal images, and that the maintenance effort to offer that is more than the community can handle today. If you want that, roll your own distro with nix, perhaps reusing the bootstrap from nixpkgs, but building all other packages above that by hand. That’s what most of the embedded space does already anyway, just typically on top of yocto.
If that’s too much effort, as the other comments say, it’s worth inspecting whether you actually need minimization of the actual images, and why. Unless you’re in the embedded space, there’s almost always a better solution than to try and hack off half the functionality of a distro to save a few thousand bucks on an enterprise deployment. That, or your use case is bespoke and important enough that you probably should invest the time to carve out your own use case, this is all FOSS after all.