What visa options do I have for visiting my fiancé in Thailand after our marriage this December?

Lots of money

Just a fyi unless you already have it sorted out. Better make sure your appointment at the embassy to get your “free to marry” affidavit doesn’t conflict with your schedule. They don’t even open up the calendar for appointments until the last week of the previous month. Depending on the district you marry and the paperwork delays it could cause an issue. An agent could prob sort out the wait though. Congrats BTW! PS, the Chiang Mai consulate is a little better at getting an earlier appointment but not by much. I would imagine they don’t carry the same staff numbers as the Bangkok Embassy despite fewer visits.

Just do visa exempt 30 day and an extension. No paperwork as long as you don't have a history of staying here

If you're only going to be visiting for 45 days, just get a tourist visa before you go to Thailand. This will have minimum requirements and will give you 60 days in Thailand. It's also cheaper than applying for an extension if you were to travel without a visa and arrive on 30 day visa exempt.