.........or I could go back to Australia, let the O-A lapse, come back on visa exempt for 45 days, ask for a 30 day extension and apply for an O longstay ( retirement) visa..
If the validity of the visa expires before the stamped in date then the stamped in date becomes superfluous.
In other words if my O-A visa expires in February and my stamped in date expires in March I will need a re,-entry permit before I leave in March and then return to Thailand at the end of March.
coming from Australia the 800,000 is only a requirement with an O visa - or 65,000 per month every month. The O-A visa only requires proof of funds(65,000 + (letter) + health insurance to the amount specified by the Embassy/Consulate for the period of your stay.
i think you may be getting confused. I arrived here in Thailand on an O-A visa on 2 April 2022 and was stamped in for 12 months. The visa date is irrelevant as long as you enter the country prior to the expiry date on the visa.