As Thailand has signed up to CRS they have access to your other countries tax returns and bank accounts. You pay the difference between what is already paid and what becomes due in Thailand due to the different tax bands and percentages. It isn’t a new law in Thailand just the full interpretation of the existing ones so nothing needs to be approved by the Thai government it’s up to the revenue department to just go for it.

You will need 0 baht or equivalent in your own private account not a business account.

Whatever you do you still have to go to the immigration office to get your passport stamped

Yes

Yes you will need proof that you are still married with a date no longer than 90 days on the day of application

You will pay the difference in tax between what you have paid in Aus and the amount due on the same amount in Thailand

Probably permission to work remotely and it will need a wet signature on the letter.

Current statement with all transactions.

Some are asking a minimum balance in your current account for at least 3 months.

It doesn’t ask you to upload a ticket or proof of accommodation so just put something in the box