I think your date choice os ideal @Jobe
For one reason or another us Aussies love to venture to the US in September, however in my years of experience in Travel I can advise that whilst crowds at the Parks will be lower on international/interstateTourist numbers, annual passes are completely unblocked (not so much of an issue in Orlando mind you), weekends tend to be loads busier, off season trading hours apply so reduced Monday-Friday and certain major attractions may be closed. (Annual Maintenance or being overlayed for Halloween in October).
May I ask where in Orlando are you staying and are you renting a car or do you plan to take courtesy transport to Parks etc? Just in my experience, WDW make it rather difficult for People staying on their properties to enjoy Orlando's other attractions.
All aside the best advice I can offer is arrive early every day and if you like a drink, try not to go too hard in the evenings.. save that for Hawaii😆
All aside sounds like a tonne of fun @Jobe! You are certainly going to be grateful for that week in Paradise at the end!