What is the earliest I can extend my TR visa at immigration in Thailand, and what are the options for extensions based on marriage?

The earliest you can extend your TR visa is when your remaining visa validity is 30 days or less. While in Korat, you can apply for either a 30-day tourism extension or a 60-day extension for visiting Thai family. For a 30-day extension, you can handle this on your own, but for the 60-day extension, you must bring your wife to the immigration office.