From Siam Legal updated 2024. This reads as if one can simply transfer 40k from an American account each month to qualify. That sounds quite lenient to me.

Non-Imm O extensions

A general enquiry - when using the lump sum option as financial qualification, can the same 400K/800k be left in the account for numerous years / applications (effectively as a bond rather than source of income) or does a fresh transfer have to be made every year with accompanied verification that it was sent from overseas ?