Also wondering if I omit my plans for Greece would it be wise to add 3 days in Prague, Czech and Vienna, Austria each ?
Having been to both I think they would be good choices. I was very glad I saw Prague, but Vienna is the one I keep returning to.
As for Greece, my advice would be if you think this may be your only trip to Europe, you should certainly at least go to Athens and see the Acropolis and its Museum and the National Archeological Museum. But if you think you'll get back there, it might make sense to save Greece for another trip, since it's a bit off your planned route.