well I don't know many cash strapped folk who pay thousands for holidays but can't find 20k cash to bring with them. Just because I don't agree that the equivalent of about £475 isn't a lot of money for starting a holiday does not mean I don't understand the value of money. What a dopey thing to say.
no need to redo maths. 20k doesn't last long. They aren't asking for evidence you can fund the whole trip. It's not much different to arriving in USA and being asked to show money, as I was asked to do few months ago.
there will be plenty of advice in this group on options to remain in Thailand as the spouse of a Thai. One is visa based on marriage, with proof of 400,000 tbt in a Thai bank. And also a retirement visa but that requires 800,000 in your Thai bank.