Can I apply for a retirement extension in Thailand with a NON-O visa and a Thai bank balance of 800k immediately? Is insurance required?

And it has to remain 3 months post extension and not fall below 0 in months 4 to 10, and then back up to 0 again prior to a further extension.

That’s in most cases.

There are some people saying at certain banks the money has to be seasoned for longer periods.

𝗰𝗮𝗻 𝗜 𝗮𝗽𝗽𝗹𝘆 𝗳𝗼𝗿 𝗮 𝗿𝗲𝘁𝗶𝗿𝗲𝗺𝗲𝗻𝘁 𝗲𝘅𝘁𝗲𝗻𝘀𝗶𝗼𝗻 𝗿𝗶𝗴𝗵𝘁 𝗮𝘄𝗮𝘆?

Nope you cannot, at most offices you can apply for an extension when you have 30 days or less left on your 90 day stamp and at some offices (Chiang Mai, Bangkok, Phuket) you can apply for an extension when you have 45 days or less left on your stamp

𝗔𝗻𝗱 𝗶𝘀 𝗶𝗻𝘀𝘂𝗿𝗮𝗻𝗰𝗲 𝗿𝗲𝗾𝘂𝗶𝗿𝗲𝗱?

No it is not a requirement

also remember you need to have the 800K in a thai bank account in your name only for 2 months before you apply for the yearly extension