What is the ruling on postponing wearing the niqab — while being convinced of its obligation — until next year without a valid شرعي excuse?

If you believe the face must be covered, then you must cover it as long as you are able. It is not permissible for you to go out with your face uncovered. A duty, when one is capable of fulfilling it, should not be abandoned to appease people or out of shyness towards them.