You won't be going to Mukdahan for an extension. They won't issue one.
You are going to the Mukdahan-Savanbakhet border to enter Laos and, on return, get a 30 day visa exempt entry.
When is your next planned visit? Whilst crossing the border in to Savanbskhet, do long as you don't mind staying over for one night, you could go to the Thai consulate and apply for a 1 year, multiple entry visa, based on marriage to a Thai.
Why is your wife, of dual citizenship, being given " a 1 year at the time"? Why is she not travelling on her Thai passport?