I'm just reading what you also trying to do. I MAYBE wrong but, someone will correct me if I am, because your embassy still issues an Income letter to tell immigration what your income is, I'm pretty certain you cannot show some money in the bank and some as income. You either show 800,000 baht in a bank for 2 months before you apply or the Income letter needs to show full income of 65,000 baht per month.
Just be aware, if you show a letter stating 65,000 baht per month, immigration MAY want to see actual proof of it.
You cannot apply for retirement until your actually 50. I'm not sure what your expecting an extension of? There is nothing on that visa you can apply for an extension, unless your married, have half Thai children etc What did they actually issue the non O visa for? Going early isn't going to work.