I don’t think it’s really about “waiting in line”. St. Therese of Lisieux became a Doctor of the Church within 100 years of her death and less than 75 years after her canonization.
The requirement to be a Doctor of the Church is that you make a significant contribution to theology or doctrine; in other words, you contributed something new. It’s questionable whether St. Louis de Montfort’s contributions were new, or whether he was simply summarizing and passing on some established teachings. I believe at one point he acknowledged himself that he was doing that.
Of course, there are those who simply say he overemphasized Mary, so his cause for doctor will have to wait for another Marian Pope to come along and move the ball forward.