Is it permissible to amend or revoke a covenant with Allah to not enjoy one's wife more than once a month, knowing that the marriage contract has been concluded but the wedding has not yet taken place, and that the enjoyment initially did not involve ejaculation?

There is no blame upon you for breaking the oath you uttered, and it is more prudent to offer expiation if you did utter it. You must repent to Allah for undertaking an action believing it to be forbidden, even if it was in reality permissible, due to the audacity involved in defying the Sacred Law.