The best part of the Non Immigrant O visa or the Extension of Stay based on married with a Thai National is that you are married with a Thai National, as long as she is your fiance you can not use this type of visa as a fiance meens not your wedded partner in Thailand. No idea if this is different in your country.
If you are not back before the last date of your permission to stay date, it has absolute no value to buy a re-entry permit as that will be valid until the same date.
Look up the requirements for a new Non Immigrant O visa based on being married to a Thai National, bring all needed copies on the trip and apply for a new visa while abroad.