How did you reached the conclusion that only a minimum amount of 10-15k per year will get you the certificate?yes this is it, thailand works if you keep the money outside the country and live there on your savings from past years.
if you are a tax resident in thailand you want to send something like 10-15k per year of income to get a tax return and pay a bit of taxes to be able to claim a certificate of residency.
also you want to check the DTA between thailand and your country of origin to see exactly how it works regarding taxes.
BTW I wouldn't keep any money or asset in thailand, only enough for living expanses. the corruption is rampant and you could get in trouble real fast there and get everything seized in the process.
And what DTA with country of origin have to do with getting the certificate?