You need the 800,000 baht deposit to get a 1 year extension of an OA visa. You can use your home country bank deposits to get the original 1-2 year visa.
As an American I just sign it myself and send it back. I’ve got my proof of life letter waiting for me in Thailand. I will return tomorrow and sign it in the next week.
I would suggest using an agent to get a Thai bank account. The rest you can do yourself if you are patient, organized, and give yourself time to to make a mistake or two
The only reason I can think of is that you want to keep your money in your home country bank and you plan to return to your home country every 1-2 years to apply for a new OA visa.