A good idea is to look outside of the automotive industry and think about what is good for the user and not necessarily what is good for the dealer. Here are a two links to check out.
If creating a thank you page feels like checking a box, you’ll be leaving post-conversion opportunities on the table.
unbounce.com
Do you even know what's on your "thank you" page? Because all of your leads and buyers do. Learn how to provide more value—and get more in return.
cxl.com
Here are a few ideas that could be good tests:
1. I think the most clear and obvious thing to add is a simple list of expectations..."Here's what is going to happen next...". And then - crazy idea - actually match the experience with established expectations.
2. Adding a video is a great idea.
3. How about testimonials..."Don't take our word for it."
4. Beyond expectations, give the customer a checklist of things to do or to prepare them for YOUR buying experience and help make it more fluid.
5. Show them customer first content like "5 things you can do to increase the value of your trade." establish yourself as a real consultant from the beginning.
6. Also consider multiple versions of thank you pages based on the source - New Cars, Used Cars, Trucks...etc. A personalized experience will always be better.
7. Showing pictures of real people is always good. It helps to humanize the process.