What are the options for an Aussie couple on an OA Visa in Thailand to manage their required funds for visa renewal?

As people have stated IF you can exit/re-enter the country before the O-A visa STICKER <-(from the thai consulate) expires and you had the proof of a year 40k/400k health insurance you would get stamped in for a whole year on a new entry stamp.

I'd certainly have the banked money ready to go IF you can't get out and back easily so you could do the extension when this stamp is running out BUT also keep a weather eye on the entry requirements to get in to the country later this year. If they ease up it might be worth it for you two to bounce out and back to get another year entry stamp and then fool with the funds the following year when you go to apply for an extension in country

You can get the best part of 2 years out of that visa before your 800,000 needs to be in a Thai bank.

Be aware that if you switch to monthly transfers you must still meet the conditions of your existing extension. So in December you get a year extension based on 800k being in the bank you must keep 800k for three months afterwards and the balance not go below 400k until your next renewal next December, which by then you’ll have a full 12 months of international transfers.

Be also aware of people pushing agents, so that you don't have to transfer the money each month. The agent merely pays a "gratuity" to an immigration officer, which is fine until you're caught out, then it's curtains!

Retirement visa 32k don't need to show any money

Your ages make this first option not doable, as you are both over the retirement age. The second option requires a full year, 12 months, of transfers, and being your extension renewals are in December, that would not work this year, but next year it could, if you start the transfers soon. Just be aware that the baht to Aussie dollar exchange rate fluctuates, and the amout required each month is in baht, so how much you need to send can change each month.