ETS Airport Shuttle (https://www.etsairportshuttle.com/) also offers a EWR-JFK shared van service that's considerably less expensive than booking your own car. I have no idea how reliable they are, but if you're not in a hurry and were thinking of taking Uber/Lyft or a traditional car service anyway, it might be worth trying to use them and you can always call Uber/Lyft last minute if they flake out.