How can I demonstrate an income of 65,000 THB for my non-O visa extension to a marriage visa in Thailand?

you don't extend it with a marrige visa -> you get an extension based on marriage.

A marriage visa does not require a monthly income of 65,000, it requires a monthly income of 40,000 baht.

You should go to your immigration office and ask them for their handout of requirements to apply for a marriage extension well before you intend on doing so.

There are only two ways to get your income approved for a marriage extension:

1) You work in Thailand and show proof of your work contract and salary over 40,000 baht per month

2) Your embassy certifies your foreign income as being over 40,000 baht per month.

Many embassies no longer offer this service, meaning the only option you have for income would be working in Thailand.

The 400,000 is in the bank is always an option, but it needs to be in your account for 2 full months already when you go to apply for your extension, so make sure you move quickly once you enter Thailand to get your account open and the money moved in if this is the option you go with.