We had late breakfast (8:00am in my case), had the car rental for 9:00am. Also did our own luggage, but our server was a bit pre-occupied during breakfast so it was rather slow. There was no wait for the shuttle, and we got to the rental place around 9:30am. I think it's safer to go a little bit earlier than later personally, when I did early breakfast I saw a lot of people waiting around since they set the pick-up time too late. So I'd probably stick with 9:00am next time too, in case breakfast ended up being quicker.