It's the first time I've bought a new car and always thought it would be an enjoyable experience and being treated like royalty spending do much money. Reality is, I've had better service and treatment buying a can of baked beans in a supermarket.
I'd be inclined to give them an absolute deadline and on the basis no car by due date then the order will be cancelled by default. As you say, spending so much money should engage their interest but afraid to say my experiences have been so similar to your so many times that I now dread the process of buying a new car!
When I think how hard I have work to secure 30 of 40 K of business, I find it hard to understand why they aren't falling over themselves when Joe Muggins walks in and spends mega bucks in one foul swoop. I've yet to meet any car sales person who I would go back to hence why I have no brand loyalty.