Sounds like a fun trip!

A couple of issues -
1. There's no direct service STL-SXM.
2. There's no direct service BNE-CHC

As you already have 16 segments, there's no room to add more. You may need to re-configure the routing or get 'there and back' to a couple of places on a separate ticket.

Rather than SYD, you should of course be going to MEL .....

When you're in SYD, make sure you climb the Harbour Bridge - not to be missed. It's not cheap to do but you won't regret it.

http://www.bridgeclimb.com/