If you absolutely, positively must buy from a dealership, you must negotiate the total price first before you talk financing, and DON'T tell them you're going to pay cash.
There are LOTS and LOTS of very informative videos on Youtube about how to negotiate most effectively.
All that said, I think you'd be better off getting a used Toyota Sienna, or as a second choice, a Honda Odyssey. Great utility, fantastic reliability.