What documents does immigration require to switch to the 40k per month method for a non-O marriage visa extension in Thailand?

All you need are two letters from your Thai bank. One just like before stating that the account is yours, and the other letter showing your direct deposits into your account every month, of at least bt40,000, for the last 12 months before you apply, and that they came from your job, or from an overseas source. Your Thai income tax filing receipt is required to extend your work permit. It is not used for your visa extension. I've been doing this for the last 8 years, and started using the monthly method three years ago. Just remember, you can not miss a month, and make it up the next month, and also that the wages must be direct deposited into your account by the company you work for. You can not deposit your wages yourself. They must be direct deposit. Keep in mind also, that after three years on the same visa extension and making more than bt40,000 a month, plus some other requirements, you can apply for permanent residency. If you weren't married to a Thai the monthly wage amount required would be bt80,000. Learn the language at a basic level, and check getting permanent residency, then you no longer deal with immigration unless you are leaving and need the re-entry permit associated with permanent residency.

if you mean a withholding tax cert - should be like this

Withholding Tax Certificate issuing by the company. (50 ทวิ หนังสือรับรองการหักภาษี ณ ที่จ่าย)

Go to your local Immigration Office and ask for their list as different offices have different requirements