We drove from Venice into the Dolomites for 2 nights at the very end of August 2013 just as you will do this September. We followed the exact route that Roberto laid out for you (both going into the Dolomites and driving from the Dolomites towards the west). We stayed in a different town/hotel each night because we did not want to backtrack at all since I'm prone to severe motion sickness and I didn't want to spend more time than I had to in the car backtracking to our hotel the 2nd night. The first night we stayed in La Villa/Alta Badia at a wonderful hotel. This area is a fairly large ski resort area and we enjoyed it.
The second night we stayed in Castelrotto at Hotel Cavallino d'Oro and enjoyed it as well though it was a totally different experience. Castelrotto seemed to be a much smaller and quaint town. We enjoyed it as well and both towns made for easy stops along our route. You said that Castelrotto "seems lovely but very overrun" but I didn't find this to be the case at all. It didn't seem "overrun" by tourists in my opinion and it didn't seem very crowded when we were there...in fact the town seemed to close up by 9pm. Most of the shops were closed and we had to search for an open gelateria for our nightcap after dinner!
I think you will love the Dolomites no matter where you stay and trust me, 2 days will barely whet your appetite and if you are like me, you will long to return again. I could have easily spent a week in the Dolomites region. Have a great trip!