Border bounce but be careful. It seems as though you already did at least one border bounce this year. You’re allowed 2 a calendar year (via land border).
UK embassy in Thailand doesn’t provide an affadavit of income so that route won’t work for you. Ask your local immigration office what the requirements are for a 90 day Non O retirement visa. It’s almost certain that you will need 800k baht in a Thai bank account before applying for the visa.
so come in visa exempt. Will give you total of 30 plus 30. As mentioned, you can apply for a non o visa retirement when you get here but be careful, a small number of immigration offices require you do seed the 800k baht for 2 months before you can apply for the visa.
Either get a 60 day tourist visa in Australia which will give you 90 days stay in Thailand (60 plus 30) or visa exempt for total of 30
plus 30 day extension. If you go 60 day visa they won’t hold you to the exact flight you enter in the system. There should be an entry on the form where you put expected day of arrival. Your visa will be valid from this date and you will normally have 90 days in which to enter Thailand on it.