That used to be true everywhere - but some (many) Imm Offices are NOT making it easy... I did it myself quietly for 12 years... then, the shit hit the fan... difficult days...
Chonburi has 3 Immigration offices - my American friend uses one - last October his renewal of his Marriage VISA was denied after 13 years... No explanation. He was forced to sign retirement extension forms and required to increase bank deposit to 800,000 Baht. My Imm office denied my retirement extension after 12 perfect years BC embassy stopped "Affidavit of Income" & I was denied use of provable deposit in home country.
FIRST: there is no such thing as a Thai 'marriage VISA'... Second: U write like Thai VISA rules are etched in stone; they aren't. The rules are administered at provincial immigration offices at the whim of their commander. The 400.000 Baht in the bank has been eliminated at some office. Third: their is no 'statement of proof of income' as I erroneously called it. The "Affadadivit of Income" previously required from home country embassies was eliminated by a number of countries. Fourth: the time your funds in the bank rule was also changed - time PRIOR TO and # of months AFTER your renewal. All that said I don't preach as an expert to anyone BC the rules change on a whim - and there are no experts for nationwide guidance.