nixos-rebuild repl --flake .#the-system should work. It still gets «error: The option systemd.services.pipewire.startLimitBurst was accessed but has no value defined. Try setting the option.»; errors but they don’t fail the whole thing. Also you can do nix eval '.#nixosConfigurations.the-system.config.systemd.units."the-thing.service".text' to get the final systemd text. Finally, you can run nixos-rebuild build --flake .#the-system to build the system without activating at all, and then you can inspect the /etc of the build configuration.
You may also be interested in Diffing NixOS configurations at the config-level although it does require patched Nix and Nixpkgs.