First of all, I agree with the other guys, it looks great.
I also agree with you that perhaps too much of the contrast is gone, however that can be fixed. Use your weathering paints that you did the rust and chipping with to now go over and "blend" some of the dust out. In other words, add some more dark paint back over some of the light, and some of the contrast will return. Dust is very difficult to model in scale, remember it is microscopic in 1:1 scale, so 1:35 scale is a real challenge. In the future, I would back if off just a bit, try to get some weathering powders, they are easier to control to model dust effectively.
Overall, your tank looks really good, and sometimes when you stare at a piece for too long, you start to lose some perspective. Remember, there isn't only one way for a model to look, you are replicating real life in a small scale, and in real life, each tank would be different and individual. I know you were shooting for a specific "look", and don't have EXACTLY the look you want, but your tank looks very good, so stay with it. A lot of modelers see a piece by someone they like, then they spend all of their time trying to replicate that model instead of replicating in your case, a Stug.
Can't wait to see the final product. Hope this perspective helps.
Scott Gentry