@Bob *********
well you can start depositing the funds now, when this marriage extension runs out you would get another one using the banked money method (400K) then after you hit 12 months of monthly transfers of the 65K baht a month you could go in, cancel the extension you're on (the marriage one) and apply for a new yearly extension based on retirement using monthly income
The tax question is beyond the scope of this group, ask on a group related to where you live in thailand sorry. .. .