You can omit the roux for a thinner soup.
You can omit the sour cream, but it will affect the flavor, the thickness and the mouth feel.
If you omit the roux, I'd keep the sour cream. If you omit the sour cream, to retain the mouth feel and texture, I'd use the roux.
You can exchange the powdered onion and garlic for fresh ingredients to your liking. Simply saute them in a separate pan until soft and add to the soup when you add the broccoli.