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.
You get a 1 year extension of your non O visa at the Thai immigration office. You can extend either as retired or as married. No need to leave the country.
You might consider keeping 800,000 in your account the two months before to apply for the next extension. If for some reason your attempt to use the monthly method is unsuccessful then as a backup you can continue with the deposit method.