What are the requirements for a Non-O extension based on marriage to a Thai wife in Bangkok using UK dividend income?

First off, you're not going to get the initial yearly extension based on marriage to a thai national by using monthly income my transfer method.. You will have to bank the money the first year..

For the second year you transfer in a minimum of 40k baht per month, each month, every month, for the previous 12 months before you apply for the next extension. All the transfers have to code in your bank as international origin.

I don't believe the immigration office will care where the funds come from as long as they're from your bank in the UK to your bank in thailand for the minimum amount every month for a year

There is no income confirmation letter you can get from the UK embassy as they stopped verifying income from abroad with notary affidavits back on Jan 1, 2019.