You should not have any problem, but they will put a "Visa Exempt" stamp in your passport rather than a visa on arrival which is not provided for US citizens.
You can enter with just your Austtralian passport and get a "Visa Exempt" stamp for 30 days. You can extend this for a further 30 days at any Immigration office in Thailand for an additional 30 days. On boarding you will need to show proof of two Corvid vaccinations or if not vaccinated then Pro-ATK/RT-PCR results (within 72 hours).
My Thai wife and I own our home and car and we udget on about 50,000 thb per month not includiing holidays and unexpected medical costs. It is possible to live on 20,000 per month or to spend thousands per month. Up to you.