Can you enter Thailand for 30 days without a visa and apply for a 30-day extension before it expires?

Be aware that your airline may insist on a ticket out of Thailand within the initial days allowed, regardless that you could apply for an extension in country.

If your ticket isn’t a changeable one you could Google “onward flights” and rent a ticket for around $15

Yes

Thai Sydney consulate website is up and going again, you could go for the TR60 visa with pre-approval online then if you decide you can then extend that when in Thailand giving you 90 days

From Australia you can indeed get a free stamp (visa exempt entry) for 30 days and extend that once for 30 days. 1,900 baht at a local immigration office.

The number of days before your stamp expires is up to the immigration office you use. Some allow 45 days, most allow 30 days prior (so with either option you could in theory go immediately upon arrival). Some like Jomtien and Koh Samui want you to apply within 7 days.

As they add the days on to your existing “allowed to stay” stamp it’s irrelevant how early you go - other than being too early in those places.

If you are from a visa exempt country then yes.