I may not be correct, but I think the bus does not start running to DTD until 10:00 or so (and even then it only runs to the Marketplace until 11:00), as most of the shops don't open until that time. DTD shares a bus with Typhoon Lagoon so it will run to TL prior to park opening.
If you have a rental car, I would suggest you drive to DTD, it's so much more convenient than the buses, which are inconsistent at best.