If you were legally married in Thailand and have a pink bordered marriage certificate then what do you mean by the marriage was never registered? You can get a Non-O based on marriage from Australia or when you arrive in Thailand (but this requires funds in a Thai bank account).
If your husband is indeed on a Non-OA Visa with a 21-Sep expiry date and not an Extension of Stay stamp that ends on 21-Sep getting new 12 month insurance and doing a border bounce before the Visa expires gets a new 12 month entry stamp.