Can I apply for an extension of my Non-O visa at Immigration before it expires?

It should be noted that a 90-day non-o visa can only be extended on the basis of the original purpose.. eg. a retirement visa can only be extended for 12-months based upon retirement.. and since the OA has not been specific on the purpose of his existing non-o visa its not clear what that is.. and unless the visa type permits an application for change of purpose this could be an issue.. and he's also not clear on what type of extension he wants.. because he cannot obtain a 30-day extension on a 90-day non-o.. and the advice given previously may suggest that is possible.. because it's been assumed he's looking to get a 12-month extension.. hence care is needed!

Should have no issuee at all applying for the extension when you get back. There's no obligation to let the stamp run down to apply for an extension and in Bangkok you can apply for an extension when you have 45 days or less left on your current stamp.

So get the re-entry permit, travel and when you come back get the documentation together to go apply for the year extension

Yes you can if you already have a Thai bank account in your name with 800k in or if your Embassy still does the income affidavit

YES... you can as planned.

As soon as you return to Thailand, I suggest you visit your local Thai Immigration office and obtain their requirements for the one year extension of stay as retired.

THEN submit your application for it.