My understanding is either put 800,000 b in the bank or show that you have a sufficientt income OR a mix of both. (The income will reduce the amount required to deposit.) So if you show that you have income it will lower the amount required to deposit in a bank. Your ss benefits verification would probably be useful and setting a thai bank account showing funds automatically transferred from ss to you bank account.
This is the way i understand it. You should go to immigration (the horses mouth. LoL) and they will tell you what you need.
Also if you live in the Bangkok Metropolitan Area the rules will be stricter than in the provences.
The only thing I've been asked at the departure airport was proof of covid vaccination. I think documents such as a forward ticket could be asked at immigration. In the 8 years of traveling back and forth to thailand I have never been challenged by airport immigration. I think its descretion of the immigration officer. Do you look like a hippy packpacker living on a shoestring?