What type of visa do I need to apply for a marriage visa extension in Thailand after marrying a Thai citizen?

Are you over 50 John and if so, why not consider applying for a retirement Visa instead.

I believe the retirement Visa is much easier to obtain than the marriage Visa (someone will correct me if I am wrong).

I think you can convert from retirement to marriage Visa a year later.

You have to remember that many immigration offices require you to be married a minimum of two months to apply for an in country non o visa based on marriage. (Bangkok is hard on this rule). So while it's all week and good to come here, get your stuff sorted, get married, you also have to stay here looking enough to be able to apply for a visa based on marriage too.

Many people enter on a tourist visa or visa-exempt, and do this

1. enter on tourist status (as above)

2. apply for an initial in-country Non-O visa based on being married to a Thai which gives you 90 days of stay, when you still have 15 days or 21 days with your stamp

3. apply for 1-year extension based on being married to a Thai when your first 90 days stamp goes 30 days or less

BUT as you will get married to a Thai person after you enter, you may not be able to apply for an initial in-country Non-O visa in time. This is because most offices allow you to apply for Non-O marriage only when you have been married for more than two months.

You may have two choices.

A: get married, correct the necessary documents, then go to the embassy/consulate to apply for single entry Non-O marriage to a Thai there

B: do border bounce or visa run for a Single Entry Tourist Visa until your marriage hits 2 months, then apply in-county Non-O

You need to go to Korat immigration office to get a handout of requirements for the initial in-country Non-O being married to a Thai when you arrive there, and check with them about details, such as their policy of marriage "months age" for application.