In PathumThani, if a retiree fails to show 800,000 baht 3 months after extension AND 6 months after extension, the punishment is "visa cancellation". I don't know what that means since the visa is no longer valid anyway, but I assume (like you do), that it culminates in deportation.
The bank book verification is on a different schedule from 90-day address reporting. I processed
my extension of stay on October 5
returned in December to process my 90-day
returned on January 5 to prove 800,000 baht in the bank (3 months after extension)
returned in March to p