Keep your address in Australia and stay with your Australian TFN. Most important thing is to retain Australian Tax Residency. As a property investor there's significant tax concessions. Don't lose them to a f*cked up tax system in Thailand
Unlikely a DTV holder will ever interact with an immigration office so it'll never be picked up. I used to do it all the time with the OA. In four years never did a single 90 day report and never got fined
A work permit ties you to one employer. I know quite a few expats and many Filipinos performing in bars and clubs. They do not have work permits, but providing the venues are paying the required "fee" to the respective police station, there will be no spot checks