It's an option, but it will be cheaper and easier if you get the non-O visa before you travel. Make many things simpler. For example if you travel without a visa you can be denied boarding to your flight if you don't have proof of a ticket leaving Thailand within 60 days.
If you are trying to live in Thailand, then you will need to apply for the non-O retirement visa before you travel to Thailand. Otherwise you will not be able to open a bank account, and even if you could it would not help you if you didn't already have the visa, because in order to apply for the visa in Thailand you must show that the money was TRANSFERRED from overseas, not brought in as cash. So apply for the non-O visa, travel to Thailand. Use the visa to open a bank account and then deposit your money after you exchange it. Then after the money has been in your account for 2 full months you can apply for the 1-year extension based on retirement.
There have been multiple reports of people with a METV saying they were interrogated and some even denied entry after trying to enter the 3rd time on a multiple entry tourist visa. Immigration does not feel you are a tourist if you are trying to stay for 6+ months.
you absolutely cannot do this. It must be AT LEAST 65,000 baht as an international transfer into your account EVERY MONTH for at least 12 months before you can apply for an extension. You should send your transfer early in the month so no delays or problems cause the transfer to slip into the following month, or you'll have lost the entire thing.
If it doesn't code correctly, you will need to go to your bank branch and ask them for a credit advice which clearly shows the transfers as international.