Note that a 90-day report is something you do when you are in Thailand 90 continuous days. If you leave and re-enter before 90 days then 90 day clock resets to day 1.
Just keep trying. There are years of reports of people obtaining Thai bank accounts on short term visas. My friend said he came here on a short term visa and tried at a dozen banks before being able to open an account. Anecdotes like this abound. I almost got rejected even though I had a fresh O-A visa. My girlfriend intervened to convince them to give me the account.
Jonathan Pressburger They are probably not looking at the details at all. I wonder if two deposits would be ok if you could back them up with proof such as a pension statement and Social Security statement.
I believe the seasoning of the 800,000 baht is two months prior to your application for a 1 year extension and not the date of expiry of your permission to stay stamp. I would deposit the money 3 months early just to be on the safe side. You probably can apply at least 30 days before the permission to stay stamp expires.