My case is exactly the same as the OP, both cases are about falling under the minimum fund requirement when applying for an extension of stay. The purpose of the extension of stay (wether this be for marriage or retirement) has no significance.
So I will offer my advice from my own experience, do not go below the required amount needed in the bank to apply for an extension of stay, no matter what the purpose of the extension of stay is.
My advice of course is only for Phuket, as others have mentioned it all depends on each immigration office.