DO NOT cancel your USA number. There is too many things tethered to your US number. I switch to pre-paid but almost never use it. Sometimes you need codes sent to your US number, keep a burner phone with you in Thailand and have your US SIM in this phone. Set up a Thai sim in your main phone. If you need to call US number use Skype/What's APP etc.
Can't you do the 90 days report a week earlier? Or Online-if you do it online it takes a few days to 'clear' so do it 7 days before and if its rejected you can go in and do it, before you leave.
For 3 months you wont need to leave the country if you get the TR 60 + 30 before you leave OZ. This fits snug with your plans, early on in your 60 days visit an immigration to get your additional 30 then forget about visas etc for the rest of your trip-