A few comments. Keeping something an open segment will trigger issuing as a paper ticket. This can cause problems later as you physically need to get the ticked changed later and this can attract fees. So it is much better to ticket with dummy dates in and then change the dates.

The fees you need to pay are a mixture of oneworld fees ($125 for re-routing etc.) and service fees by the airline. The later fees all depend on which airline is making those changes and often are higher for paper tickets. So if ticketed through AA, then usually there aren't any fees.

Typical starting points at the moment are Indonesia and South Africa. Even NZ is cheaper than Australia (which I think is the most expensive country on the planet for a DONEx). Just play with the website.

Also, your routing doesn't really doesn't maximise a DONE4 at all. You can easily get double the amount of points/SC out of a DONE4. In fact, it might be cheaper to buy some individual tickets for this but then this might not be on OW.