Since youre limited to five stopovers, you can do something like SYD-LAX-DFW-PDX//YVR-JFK-CDG//LHR-JNB-HKG-BKK-SYD with Bangkok or BNE-SYD-LAX-DFW-PDX//YVR-JFK-CDG//AMS-LHR-JNB-HKG-BKK-SYD-BNE without. On the first routing, you would need to buy a ticket BNE-SYD-BNE, a train ticket from Paris to Amsterdam to London, and then a ticket from Portland to Vancouver. The second, you would just need the train ticket from Paris to Amsterdam and the PDX-YVR ticket. I am sure that others will come up with something comparable or better.