Incorrect.
2 months prior to application for extension and maintained for 3 months after application.
Not below 400k in other months
Edit: from AI ..
"To extend a retirement visa in Thailand, you need to maintain a bank balance of at least 800,000 baht in a Thai bank account for two months before applying, and then for three months after the extension is granted, and ensure the balance doesn't drop below 400,000 baht"