What date determines the start of the 3-month seasoning period for a visa extension in Thailand?

Two months of seasoning (before application) has to be fulfilled when you apply for next year's extension, not the date of expiry or the last extension application. So it depends on what date you are going to apply for the next yearly extension in 2022.

For example, If you are going to apply on the very last day, you put 800k in your bank account on 12 July. If you go to apply on 15 Aug, you put 800k in your account on 14 June.

Your fund has to be in your bank account for three months (after granted extension) from the extension started, not the date you applied for an extension. Seeing your stamp, you need to keep 800k until 13 Dec at the earliest.