Extension of stay based on retirement

IMOif you can meet the 650000 baht income requirement it is better than having the money in the bank.
No need to worry about having the money in the bank3 months before the date you do the application or the chance of something the balance below 800K baht. Also you can keep the funds in any type of account that may have a higher return.
Even going to the embassy for the income letter is not that big a deal since it can be up to 6 months old on the date you apply for the extension. You can plan a holiday trip during the 6 months to get it if you want to.