When should I apply for the one-year extension of my Non-O retirement visa and what is the reporting process?

You DO NOT do 90 day reporting until AFTER you get your year extension. That's when they will put a small paper in your passport showing when your first 90 day report is due Like this

you can apply for the one year extension when you have 45 days or less left on this new 90 day stamp from the Non-O visa. So if you have a stamp expiring on March 20, you can apply any time AFTER Feb 3rd.

As long as you have your 800K baht seasoned in a thai bank account in your name only for 2 months before you apply you're good money wise.

This is a general idea of what can be required to get a year extension based on being over 50 (retirement)

You do the one year extension when your non O is “winding down”. Some offices allow 45 days prior. Most 30. You don’t do 90 day reports until you’re on the year extension. When you get it they’ll put a slip of paper on your passport to show what date your report is due. [edit] Missed the part where you mentioned Bangkok. CW allow 45 days prior.