Like some that have posted I always start with a roux, but have never used cornstarch as a roux but regular white flour.
The more that you brown your roux it lessens the thickening power howver...learned that from Emeril Lagase. I dont brown my roux but cook for 1 full minute to remove raw taste.
When I make my cream soups Iuse stock, to make the gravy but finish off with half and half or full on heavy cream. Have used plain milk in a pinch.
Thanks for the hint on rice flour...i'll give that a try.