You will not be applying for a Marriage Visa in December, you already have your Marriage Visa so it December you will be applying for a 12 month Extension of Stay based on marriage.
Not 180 days, it's 60 days per entry then you either extend for 30 days or leave and re-enter for a new 60 days. With proper planning you can get close to 9 months stay in Thailand with a METV.