I am not questioning the dogma of EENS. I don't know what the reward of the just consists of. I suspect it is Paradise, aka "the New Earth" discussed in Apocalypse 21-22.
Adam, Noah, Moses, Isaac, Jacob, Abraham, the Good Thief, St Joseph -- these were all "just" men because....a) they believed in the coming Redeemer, b) practiced God's religion and c) were circuмcised (if applicable).
Plato was not "just".
How do you
not know what the reward of Adam, Noah, Moses, etc is?

The Old Testament "saints" are surely saved.
All of the OT "Fathers" that were in "Abraham's bosom" were unbaptized. They died before the Sacrament was instituted.
Of course they were unbaptized. But the fulfilled the OT Law requirements ... a) they believed in the coming Redeemer, b) practiced God's religion and c) were circuмcised (if applicable).
Plato did not fulfill the Old Testament Law. He was not an Israelite, but a pagan Greek. He may have lived a "natural law good" life, but that's the extent of it (there is no record of him converting on his death bed to Judaism). Assuming he didn't convert on his death bed, he cannot go to heaven. He cannot be called "just".