Rob, nice image mate you have some great detail.
Like you I am on the northside of Brisbane, with the same dreaded light pollution. I use 1 minute subs at an ISO of 800, then get lots of them
You have a big light bucket there, no need for high ISO and long exposures. You can get just as good with short subs, data is data.
Getting around double the amount of subs you have will allow you stretch that data a little more and keep the noise down.
Clear skies
