Can I apply for a retirement visa in Thailand based on monthly income instead of an 800K baht deposit?

800k.

You simply transfer a minimum of 65k from overseas each month. I've done it for years

You can do so in a consulate outside of Thailand for the initial 90 day Non-O visa but to get the 12 month extension you need 800k baht in a Thai bank for 2 months before you apply

Unless your embassy issues an income letter you can’t not on the first round

How? You don't. You have only 2 options to apply for a non-O visa based on retirement in Thailand. Proof of a Thai bank account in your name with 800,000 baht, or embassy certified income. If your embassy does not certify income, then that means you are left with only one option.

The same goes for the first annual extension, you either show the bank account balance, or embassy certified income. If your embassy doesn't do that, then the bank balance is the only option.

For the 2nd annual extension, you can use monthly international transfers into your account to satisfy the monthly income requirement. But that will not work for the visa (if trying to apply in Thailand) or the first annual extension, even if you had the Thai bank account for 10 years with monthly transfers, it is not accepted.

Bank letter, bank book, and statements .