The sacramental union survives termination of the civil marriage and endures until dissolution by ecclesiastical divorce.
So what you suggest is 1) the Church requires first a civil marriage, thence 2) institutes a Sacramental marriage. Then if 3) the civil marriage is terminated, the Sacramental marriage remains and finally 4) the Sacramental marriage is dissolved by the Church.
What about the case where a civil marriage existed previous to conversion? That is not uncommon among converts. In those cases, are you suggesting that the marriage is sacramental?