maralorn: One last issue:
It doesn't actually look like I have the HLS for 9.12 in this dev environment.
I'm getting this error from the Emacs LSP mode:

haskell-language-server exe candidates: ["haskell-language-server-9.12.2","haskell-language-server"]
Failed to find a HLS version for GHC 9.12.2
Executable names we failed to find: haskell-language-server-9.12.2, haskell-language-server

Using this shell.nix:

{ pkgs ? import <nixpkgs> {} }:
let
  src = pkgs.nix-gitignore.gitignoreSource [] ./.;
  myPkg = pkgs.haskell.packages.ghc912.callCabal2nix "aoc25" src {};
in
pkgs.stdenv.mkDerivation {
  name = "aoc-shell";

  buildInputs = [
    myPkg.env.nativeBuildInputs

    pkgs.cabal-install
    pkgs.haskell-language-server
    pkgs.hlint
    pkgs.ormolu
  ];
}