Since he asked the question in regards to leaving Thailand and returning home, it is irrelevant. There won't be a next extension application and that's when the problem with the money being gone would become apparent.
Do you have the 90-day Non-O VISA, or are you on a 1-year extension of stay based on retirement? It makes a HUGE difference.
You said your non-O is "registered" in Phuket. What does that mean? If you are on the 90-day non-O visa, did you apply for that and receive it from Phuket, or did you get it from an embassy before you traveled to Thailand?
Citizenship is not an option for most people even if they are married to a Thai. You have to work in Thailand for multiple years while paying taxes to qualify, and then you must continue working the entire time the application is processing (more years).
1-3 weeks depending on the embassy. There are 4 in the US and the one you use is determined by where you live. The visa is valid for 90 days so you can apply for it anytime within 90 days of your trip, but normally 1-2 months early is plenty.
Yes, the non-O 90 day, extended annually with 800,000 in the bank is the "normal" way. It'll get you what you are looking for. There are more advanced things you can do such as switching to monthly bank transfers as proof of funds after your first year, but they require planning a year in advance and making sure you never miss a transfer.
Just make sure you get the non-O visa from the Thai embassy BEFORE you go to Thailand, or you won't be able to open a bank account. No bank account, no extensions.
If it's the same bank account or bank that you transferred the money into, should be as simple as a transfer out. If you've changed banks, you'll likely need to show them the source of the money when you transferred it into Thailand.
It's a general travel rule in general, but it's not the rule of every country. If you want to avoid issues, follow the rule. But if you get to Thailand they will let you in without it.