Achaemenid art was not very innovative. In the days of Darius I the Great, the basic forms were established, and later artists did not really change these patterns. A king was shown sitting on a throne (example), or killing an animal (example), or sacrificing (example). On seals, there is some variation, but essentially, the Achaemenid artists preferred to emulate good art instead of inventing something new.
As a consequence, they never invented the portrait, and all kings look like Darius, with the same beard. Until now, I knew only one representation of an Achaemenid king by an artist who wanted to show what the ruler really looked like: the portrait of Artaxerxes III Ochus in the Amsterdam Allard Piersonmuseum, made in Egypt. We may perhaps add the Darius III Codomannus on the Alexander mosaic, although I can hardly believe that the Greek painter whose design was used as a model, had really seen the great king.
In the Archaeological Museum of Antalya, I discovered another candidate: Artaxerxes II Mnemon is represented on the tomb of (probably) the Lycian leader Pericles of Limyra. Unfortunately, it is very damaged, but the man clearly has his tiara tied up so that it stands erect. You can also recognize the diadem. Did the sculptor really see the great king?