Is there a passport validity requirement for renewing a Non-O retirement visa in Thailand?

There actually is NO requirement at all as far as passport validity to get a new yearly extension at your immigration office.

Works like this

IF your passport isn't valid for the full year you would get when you apply for the extension you only get the extension issued until your passport expires. 😮 Then you'd get a new passport and apply for a new yearly extension on the new expiration date

As far as your question your extension if you apply for a new yearly extension at your immigration office in Dec for an extension that runs out in Jan you'll get a stamp good until Jan 2027, Your passport is valid longer than the extension you'll get