I am planning to visit on september and do the 9:30am tour, however when I checked online there is no more ticket for the 9:30 am slot (bummer), is 10:30 am or 11am slot ok to do? or would it be better if i go around 2pm?

Welcome to Fodors. Asking questions about your own trip - it would be better to start you own thread.

But never the less, I checked 10+ random dates in September and there were no sold out time slots.

It only depends how long you want to stay in the Castle. Once entered, you can stay there until the end of the day. Four hours is more than enough.