Hi unfortunately most of the beaches require a climb to get to them.
Third Beach at Coolum is probably not a good idea as well as there have been arrests.
Sunrise is generally not recognised as nude. A/Bay while not legal is about the only beach where nudity is tolerated.
Brisbane Forum run swim nites on a regular basis. Give them a yell!
There is a place called Balkaz near Caboulture. You will find a lot of info in TAN magazine