Extensions (other than your first) are irrelevant for 90 day reporting. The “clock” starts when you enter Thailand and you report if you’re staying over 90 days.
Most offices will allow you to apply when you have 30 days left on your current stamp. Some like Bangkok Chiang Mai and Phuket will allow 45 days prior.
Unless you’re applying through the Sydney consulate it’s probably not a visa you’re going to get or qualify for. There have been instances where people have got one but very few and far between unless using Sydney.