You can manage your Emacs packages with home-manager, it looks like this:

  programs.emacs = {
    enable = true;

    extraPackages = epkgs: with epkgs; [
      ace-jump-mode
      ag
      auto-indent-mode
      browse-kill-ring
    ];
  };

and here a function that builds a package that is missing in MELPA:

  build_movelines = epkgs: epkgs.trivialBuild {
    pname = "move-lines";
    version = "2.0";

    src = pkgs.fetchFromGitHub {
      owner = "targzeta";
      repo = "move-lines";
      rev = "master";
      sha256 = "0pbvn11n9sw3mg3hpcpc8p80zh53sxyjbhqqxdwbwg8dwpc7r5ij";
    };

  };

Guix works as a package manager for Emacs, though you’ll always need to maintain your init.el

straight.el works too of course but ultimately Emacs configuration is not declarative.

There is also nix-straight, which is used in nix-doom-emacs, although it is fairly limited and takes ages to build when you make any changes.

As I understand it, that's basically what this is.