When should I start the process for a Thailand retirement visa before traveling to Chiang Mai?

You can apply for the Non Imm O in Australia, you get 90 days to enter Thailand from it's date of issue.

Easiest method for you is going to be entering Thailand on a Tourist visa or visa-exempt, then converting to the Non-O retirement visa in the country.

Tourist visa is valid for 90 days from the day it is issued, so you must enter the country within those 90 days and then you will get stamped in for 60 full days.

Given the dates you mentioned, you can apply for your visa later this month while you are in Australia (since you are supposed to be in the country you are applying from) and then worry about the Thailand Pass once you're comfortable in Cambodia.

You also have the option of just not even caring about visa for Thailand and entering on visa exempt which will get you stamped in for 30 days.