that will be rush hour time beginning. You may want to take the train into town to avoid traffic. Have you been to Bangkok before? The Grand Palace apparently opens at 8:30 so you would have time to go down and check that area out.

Are you flying economy or up front? There have apparently been some issues with time to get through security, but if you're in TG C or F there is a dedicated fast track security. But there is something to keep in mind. If you have checked bags that may end up being an issue for you as well unless you get your bag checked to your final destination.

If you are on separate tickets it's likely you will have already paid the departure tax.