It's not that difficult to get a retirement visa/extention. It is actually the easiest visa to obtain without alot of hassle. All you need is money in the bank (800K) or confirmation of 65.000 Baht monthly income. A leasing contract for your recidence, and your passport. Done in less then a day
Just be prepared if things go sour, you will not be able to demand anything. I know many guys who has spendt millions renovating or building houses in Isaan, and when the marriage /relationship is over, they do not have any rights. So spend what you can afford to potentially loose, without you becoming broke is things turn bad