pensions both private and governmental are highly favored, as is substantial investment income, or trust funds, any documented income will do fine. They are concerned that the income is stable and on going through out the visas validity. Digital nomads that can generate income anywhere have their own visa category.
If applying in Bangkok the ship of state generally follows the official rules. One thing you can count on is the rules will change. David hit the bullseye. Provincial offices occasionally tend to assign different priorities to the requirements.
I've used the US Thai evisa system twice. If it's the same software the first time is frustrating, but the system auto saves your application, and you can take it in digestible chunks. Passport photo file size is limited, and uploading some files was a bit clumsy. Approval time was quick. Overall I prefer it to mail in.
Visa runs can be fun. I've done at least 15. Take the train to the nearest border crosing, (Nong Khai is a convenient crossing point for me) stamp out of Thailand, pay for a visa on arrival then re-enter Thailand...or since you paid for a visa take a short holiday. Myanmar border isn't cool now. Cambodia-Thai border at Poipet is full of hustlers, and there are some dicy characters at Cambodian immigration. I haven't used the Malay border. Lao PDR is cool, or Vietnam is a short cheap flight; inexpensive rooms and great food.
As a general rule the farther from Bangkok you are the less hassle it is. Different provinces seem to have a slightly different interpretation of the minor rules. Proof of income, residence address, and bank deposit requirements are universal. My friend in BKK had to submit two photos taken from the knees up, wearing different shirts on his last renewal 555.
the ฿800,000 can be ฿400,000 if you show the other ฿400,000 in income. If for any reason one wishes to repatriate their money only ฿50,000 can be taken out of Thailand per transaction.