Lowballs are to be expected. Was looking at an M3 as well at Tischer, and they offered me $16K; granted, my car has 13,000 less miles than yours does. Trade in values don't come anywhere close to what you think your car should sell for, which is why many members sell them directly on here, or via other social media to see if they can get a better deal. Its just the way shit works. I also think race and age may have had to do with something in my case, but I won't speak anymore on that; it is what it is.
That said, I have decided to keep the 335i and look elsewhere once the F80 M drops, as the previous models are now outdated and the E90/E92 M's cost far too much for what they are, IMO. You definitely made the better choice in keeping your car.
As for the service reps, they are a completely different story. Jack Barber has taken care of our cars since 2005 when he worked at VOB, and he is now at Tischer. Best guy to have on your side!