Metros everywhere else in the world (Europe, Asia) are designed with stations very close together, some within 5 walk of each other. The point is you have high density all along the route so most people are only taking part of the journey, not from end to end.

And if you want 20 min between Parramatta to Sydney, the best way is to have a metro with really close stations, and have heavy rail only stop at Parramatta, Strathfield and Central. Have a metro serve all the intermediate stops.