Does an SSI letter indicating a monthly income of over $3,300 replace the need for an 800,000 THB bank account for a Thai visa extension?

No

The United States government does not want you spending your money in another country they’re gonna do everything they can to get you to spend that money back home!  they will not certify any type of letter and that’s all that is accepted and the US government knows this

You'll need 800k thb for the first year regardless. If you make international transfers monthly above the minimum requirements you can switch on year 2, however make sure you transfer more then enough to account for currency conversion shifts.

The US Embassy no longer gives those verification of income letters.

No, you still have to show the 12k or 24k US in savings good idea to have 6 months of it and when you apply have statement that is new to reflect it.

They dont care about your income they want to see you can manage finances and can support yourself and not check to check

When you initially apply for your type O 90 day visa based on retirement on the e-visa system in the US yes it does work, but after you arrive in Thailand on that 90 day visa you must get a Thai bank account and put the bt800,000 in it at least two full months before you apply for your one year extension of stay. The US embassy no longer issues wage statements so you must use the bulk sum method for your first year extension. There are no exceptions to this. By getting your type O 90 day visa before you arrive in Thailand it makes it very easy to get your required Thai bank account once you get here

hard no

It's easier to put the 800,000 into a Thai bank account and forget about it.

No. The only thing immigration office accepts is embassy certified income. The US embassy does not do this anymore for many years. So income is not an option for your first extension. You can use 12 months proof of monthly international transfers for subsequent years, but the first year will always be 800k in your Thai bank account.