You've been thru the process already can't you do again what you've already done once. Keeping an eye on the latest changes and updates re CoE and Sandbox/Quarantine..
Generally speaking 3-4 weeks from your departure date is plenty of time to apply for the visa. If you still need to get the CoE when you go, you'll need the TR Visa first before you get the CoE.
Get the TR visa in OZ and when in country there is many options to stay longer. I am doing the same thing. The TR visa can be given to you fast b/c not many Australians applying for VISAS or leaving. Even by post; Melbourne->Canberra I got mine in 8 days including w/e. If you do go Post use Express Post envelopes both ways for next day delivery, this will save you postal time.
You should be able to find a fast result 3hour turn around PCR test in your city. Costs more but good for peace of mind, with all the 72 hour math etc.
Agreed don't get married just so you can stay here longer. How long have you spent in Thailand previously? Can you test the waters and stay a year or so on visas and then think about other options to stay long term later?