That's up to you. But if you are able to open Thai Bank Acct. and make 800K Deposit, it will then be easy to simply obtain your Retirement Visa (Non-O) after arrival -- probably with less "fuss" than trying to obtain from Embassy abroad. Either way, you must then apply for the 1-yr Extension after your arrival, with basically same requirements. Again, if you want me to Post instructions, just ask. Otherwise, I won't.
If it would be helpful to you, I can Post simple do-it-yourself step-by-step Instructions for obtaining the Retirement Visa, after arrival on a TR or Visa Exemption. Just ask.
Many/most of the major Travel Websites also allow cancellable/refundable 24hrs: Kayak, Expedia, Travelocity, etc. BUT, of course, this won't work if applying for a Visa, TR or otherwise, as application/approval process would take far longer. As for use with Visa Exempt, time will also be tight -- must book shortly before departure, and hope you arrive without major delays, and can cancel after pass thru Thai Immigration.
That's not what I said. Of course, you can switch. But, I said if you do switch from Monthly Method to Bank Method, then you are simply in the same boat with money "tied up" in Bank Account. And, you would most likely have incurred more in transaction fees, than simply doing lump-sum in first instance. So what would be the point? Finally, if you are USA, UK or Aussie, you could not even do the Monthly Method the first time (which is what the OP/Author was asking about), since you are required to have proof of 12 monthly deposits for the prior year. So, I guess I wasn't "wrong" after all. 👍
If you are not working and applying for Non-O based based on Retirement, then I believe the answer is no. Wife would have to wait until you are actually on the 1-yr Extension, for Dependent app. inside Thailand.