The Deal is you're believing the wrong people.
The rules changed back in Dec 2018 for the seasoning of the funds for the 800K baht banked money method to 2 months before you apply, the funds stay in the account for 3 months after you get the extension and then they can't go below 400K baht the rest of the year.
As
@James *******
already told you in the thread I deleted; the person giving you advice is confusing getting a Non-O VISA issued inside the country (when you need at least 15 days left on your current stamp) with getting a year extension in side the country when you already hold a non-O visa <- which you can do right up until the last day left on your current 90 day stamp if you wanted to.
Most offices let you apply for a new extension when you have 30 days or less left on your current stamp (some will let you do it when you have 45 days left). They add the new yearly extension to the expiration date you have, so if you meet the requirements there is no reason to wait until the last day. You aren't losing any time applying early