I mentioned that a recent TM30 was needed. If you don't have one you need to get one, possibly by the things you mentioned. However, you would already have been around for a while and would already have one.
Again people are conflating income or bank. When someone shows up for an income extension it would mean they have probably been in in Thailand for at least a year and you would have 800 / 400 ฿ in the bank. So it is very plausible for immigration to ask for proof if that. I never had ฿800 in the bank for more than a few minutes, hence I was concerned.
if you have at least ฿65 K in your home bank just transfer to a Thai bank every month. No one has ever asked me if it comes from retirement, just that it is a foreign transfer.
for income method, 800K 400K only comes into play in that you need a visa for depositing the first 12 deposits. You start for the first 15 months and get your 12 deposits then switch. Do not try to think about the total just that you have at least 65K X 12
wrong, did you read the op? 800,000 has nothing to do with it for the income method. You are talking about the money in the bank method. In fact most people do the income method precisely to avoid the 800,000