22/24 Sept is in the first week of school holidays, but I wouldnt think too much effect.

Arrival times being Fri approx 5am and 7am. The former makes the 7am flight to SYD a possibility, the latter 9am.

Both likely to be full in business, perhaps more chance of seat 9am (the 7am arrives in time for start of day in SYD so gets more business traffic). However the early and mid afternoon flights would be the best shot if you're okay waiting several hours. Late afternoon again will be full in business.

The other consideration is 7am and 3:30pm flights are in the 767 whereas others are A320. The 767 has 24 biz seats vs only 8 in the A320.

I would go for 7am flight as might get lucky with someone cancelling and free up a seat, or 3:30pm more likely but several hour transit (vs 1h45m for 7am).

Hope this helps.