What are the requirements and timing for applying for a 1-year retirement extension in Thailand after an O visa?

Two months before and three months after not dropping below 400k for the rest of the year

you got no other options.

When this stamp runs out, if you wanna stay longer, you bounce out and back to get a 45 day visa exempt entry.

I'd certainly keep trying to open a bank account (you just didn't try enough branches and it usually takes 10 to 12 tries to open one)..

That way the next time you can get the Non-O visa again and come back and that time you can get your year extension.

I suggest maybe you consider using up your current visa and doing a border bounce to get 45 day visa exempt stamp plus a 30 day in country extension. When you return in the fall come in on a tourist visa or visa exempt and then apply in country for a 90 day non O, then your extension after that. Consider your travel timing carefully so that your non O extension coincides with your yearly November or April travel schedule , if that's your ultimate plan kinda thing.

Your timing is going to be short as the visa and extension will run out in March rather than end of April. Unless you have a Thai wife or family, then you can extend 60 days due to Thai family, which would put you in the timing ballpark and apply for the year extension after that.

Money must be in the account a full 2 months before you apply for the extension.

you need the 800K baht in a thai bank account in your name only for 2 months before you APPLY for the yearly extension based on retirement.

So if you have a stamp expiring on the 7th of March you needed to have the 800K baht in a thai bank account yesterday to hit the 2 month seasoning before your stamp runs out. 😮 <- and that'd be if you waited until the last day of your entry stamp to apply for your year extension.

Realize that you want to stay until 30th of April, doesn't mean you CAN on the entry you have 😕You might have to bounce out and back when your stamp is expiring to get a free 45 day visa exempt entry.

As far as does a yearly extension make sense for you, only YOU can decide that, no one on this group can tell you.