I've been using Emacs overlay to compile packages ahead of time for a while. Unless I've misunderstood how it works, that's what the variations on emacsWithPackages do.

I use the version of emacsWithPackages that parses use-package declarations from my .emacs file and uses Nix to compile and install the corresponding Emacs packages. The upside is that this also pulls in native dependencies (pdf-tools/etc) but the downside is that adding a new package requires rebuilding at the Nix level and restarting Emacs. It has not been a major problem for me—I don't install new Emacs packages very often at this point—but I can definitely see how it could get annoying for anyone who does!