@Bonnie ******
Better to try to make the deadline. You've analyzed the situation (which method is cheaper in this case) well and it's time to pull the trigger, in my opinion. I would make August 31 your deadline, as that leaves 15 full calendar days before your current permission to stay expires. If you initiate the transfer today I think you have an excellent chance of having the transfer complete successfully by or hopefully before August 31. Don't delay with the transfer would be my advice.
The covid extension is a possible plan B but I hope and expect it won't be needed if you proceed in a timely manner with the transfer.
Obviously, you can do almost all the paperwork for the non-O visas beforehand and only need the bank letter from the bank.
Also, unless you have specific guidance from the immigration office you plan to use, the guidance from the Phuket Immigration Volunteers is pretty comprehensive. I would take a look at that if you haven't already or have better information from your local office:
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