Get a non o visa (90 day) before you get to Thailand Open bank account deposit funds (400k thb for marriage to Thai or 800 k thb for retirement) go to immigration before 90 day visa runs out an apply for 1 year extension based on marriage or retirement
Personally I think retirement is easier as marriage has lots of fuss to prove that you are really married home visits etc Good luck
No need for an agent If you follow the advice given by others in your post you will be fine Australia is not as super picky as some other countries Good luck