I run several personal NixOS servers. it's definitely doable without custom expressions (I can't remember the last time I had to write one).

lots of services defined in the NixOS options will have an "escape hatch" option named something like `extraConfig`. this usually lets you append verbose text to a complex config file, like in the case of Postfix where the options couldn't conceivably cover everything.

you can also always just write a static file to /etc if you want or need to:

    environment.etc.foo-config = {
      target = "foo.conf";
      text = ''
        enable_spline_reticulator = true
      '';
    };

as well as running one-off systemd services declaratively:

    systemd.services.foo = {
      after = [ "network-online.target" ];
      wantedBy = [ "multi-user.target" ];

      serviceConfig = {
        ExecStart = ''
          ${pkgs.foo}/bin/food \
            --config /etc/foo.conf
        '';
        User = "foo";
        Restart = "on-failure";
      };
    };

(both of those examples are copied more or less straight from my daily driver NixOS 20.09 laptop before I made the details generic)

another useful escape hatch is that you can run Docker/Podman containers declaratively. I have some things I want the flexibility to upgrade on a different cadence than NixOS itself (such as my family's "production" Jellyfin server) that I just run as standalone Docker containers with volume mounts for their data.