Turboprops often cruise a lot higher than what you quoted. More accurately it’s unpressurised aircraft that cruise below FL200 as a guide. The RFDS/RAAF king airs have a surface ceiling of FL350.
There are quite a few options ex BNE to SIN for 74k each in J and two in J for 90K each SIN-BCN, coming back is more problematic. You might find it better to use points to get out of Aus and grab a low cost J fare ex Asia
Even with luggage I suspect bus will be not a bad option Sydney Airport Domestic to Bondi Junction | transportnsw.info. When I lived at Earlwood I hated grabbing a taxi home, and half the time the driver didn’t know where Earlwood was!