There are a lot of boats an Chalong Bay Phuket that have done the reverse trip. No doubt a few beers at one of the three self appointed yacht clubs at Chalong will get you introduced to multiple yachties who have done your trip in reverse.
Have a read through the below writeup on Noonsite detailing sailing back to Oz from South East Asia. Lots of detail and OpenCPN routes are available as well I believe.
In this writeup it clearly outlines what the choices are, and details one person's experience and recommendations. I think it is a case of picking the least worst option, and perhaps depending on the year it may be a bit of luck in terms of what works.