The short answer "is it worth it"
No - You buy a Business Class Return from Lon- Syd for around £1900 on SQ -
this is about £400 less than what you pay For a DONE4 (Business Class)
Before you start adding on taxes will make a bigger Diff £600 +++
If you want to travel the WORLD and not Syd - LON and Asia
Then yes get a RTW
The price gap for F class is more in favour of going to CAI