What are the best options for Aussie retirees on an OA visa in Thailand regarding their bank funds and visa renewal?

As you can't border bounce to get your second year of the O-A, you'll just need to keep your money in the bank to get a one year extension at immigration.

Your option this year is use banked money method on BOTH of the O-A extensions in Dec, THEN start transferring in the monthly 65K baht a month for the next year to qualify for the monthly income method next year.

Sadly doing this means you need to meet the seasoning requirements for BOTH proof of funds methods. You need to keep the 800K baht in the bank accounts for 3 months after you get this extension granted in Dec, AND then you couldn't let the balance go below 400K baht the rest of the year <- those are the seasoning terms on this extension you'll get in Dec,

AND

You'll have to bring in 65K baht a month each month every month for the previous 12 months before you apply for your following extension too.

Switching proof of fund methods especially banked to monthly income are troublesome.

You can't switch OFF an O-A entry stamp to an extension based on being married to a foreigner on a long stay visa/extension (piggy-back). You have to get at least ONE extension for the reason you got the visa (retirement) before you could change the reason. So piggy backing is out

You couldn't switch to the monthly income method this Dec when you need to apply for a yearly extension because you need to transfer in 65k baht a month, each month every month for the previous 12 months before you apply for your extension. So monthly income method is out

so you're still on the original year entry stamp you got when you came in on the O-A visas right?