Hi and welcome to this humble forum.
The real trick w/SYN is returning to US. Some folks here have done it. Granted, her DOH is pretty nice but it's still a buddy pass. Since, you're flexible, you may be just fine. Not sure about any Aussie holidays--might want to check first. They have what they call "schoolies" kinda like our spring break but they don't follow the same seasons.
Check out OZ tripadvisor for general info on holidays.
When you get closer to dates, pls specify and we can check detail/chances.