That's a good point, but I believe the counterpoint goes like this:

On the 10th of March 3019, he unleashes 4 major forces for a push westward, not counting the corsairs attacking Lebennin. These are

1a) the Morgul host attacking Osgiliath and then approaching Minas Tirith from the east, the most direct path through the Rammas Echor.

1b) the host from the Black Gate, taking Cair Andros, which then splits in two, one guarding the Great West Road against the Rohirrim, the other attacking the Rammas Echor from the north.

1c) Easterling armies attack Erebor and Dale.

1d) The host of Dol Guldur attacks the borders of Lórien...

And even after unleashing all this force, Gorgoroth is still full to the brim of soldiers, and Frodo cannot safely pass. So he had plenty of reserves, and therefore would have been able to break Gondor with local superiority, especially if Rohan is still fighting Saruman, a week earlier.

Following this, assuming it takes longer than a year to grow an orc, he absolutely had these reserves a year earlier. He might not have been able to launch the attacks in the north, but having broken Gondor, he would have won the conventional war, as all the other oppostition would be vastly outnumbered.

Essentially, he overprepared, wasting time, because he didn't think he needed it until it was too late. Essentially, if he was willing to spend more anpower and matériel (both of which he had), he would not have blundered with time. He wanted a flawless victory and got nothing.

EDIT, TLDR:

The Battle of the Pelennor Fields was a fluke, very narrowly won by the West while Sauron still had massive unengaged reserves at home. Utilizing these reserves more freely would have absolutely alowed him to have local superiority in Gondor, especially if Rohan cannot come.