How soon can I request a 60-day COVID extension after arriving in Thailand via the Phuket Sandbox?

Whether you can or can't get a 60 day covid extension before the 27th of Sept (the date the current program expires) is going to be totally up to the immigration office.

As you're not even here yet, and won't be until the 19th I'd say head on over to the immigration office the week of the 20th and see what they say. it's all you can do.

There is no visa on entry (45 days). There is a free visa exemption entry for visitors from certain countries for 45 days and I assume that's what you are coming in on.

These can be extended for an additional 30 days.

After I entered on a 60 day tourist visa in May 2021, I was informed by Immigration that I had to apply and pay for the 1900THB 30 day visa extension first and only after obtaining this visa could I apply for a 60 day 'unable to travel due to covid extension', despite the covid extension being in existence at that time.

If Immigration doesnt extend the covid extension for a further 2 months beyond 27 Sept which is what is being rumoured, you will be res[rice's to 45 days + 30. If you wanted to stay beyond that, the only options available would be to apply for an alternative visa such as an Ed visa upon your arrival.

BTW... Be sure to the hotel has filee a TM30 address registration... If you go to Thai immigration as soon after you arrive in Phuket. And again when you move from the hotel at end of the 14 nights...be sure to have a TM30 on file with Thai immigration.

Your situation is unique as you are entering the Phuket sandbox.

Timing is key to obtaining the COVID extension... Assuming it's not extended again past September 27th

Ask Thai Immigration in Phuket. There is no guarantee they will give you the Covid extension.

As you are entitled to a 30 day extension. They may require you to use the 30 day extension first.