The Tolkien Bestiary is considered absolute garbage as a resource. I have a copy and it is. There isnt really any solid answer and we dont only see goblins as the military forces of sauron at all. That is explicit in the two towers when orcs from morder, isengard and Moria are in deep dispute over what to do with Merry and Pippin. The orcs from mordor wanna bring the Hobbits back to the nazgul.om the other side of the anduin that Legolas shot down, Saruman’s Uruk Hai wanna bring them to Isengard and the Moria guya were just out for revenge and wanted proof of a few kills so they could go home. Their is also the orcs in the tower of Cirith Ungol talking about just fuckimg off into the mountains to just be normal bandits instead of being wrapped up in all this.

There are issues with how the creation myth written in Ainulindale works and your explanation as well. It just doesmt seem to be how the world works