Basically, there are some minor issues with the Plate Shields. Volcanic Plate Shield pretty much outclasses Ancient Plate Shield and Ironmight Plate Shield outclasses Royal Jelly Shield but is still often left in the dust compared to other things. Either way, onto my suggestion:
Why not just make the Plate Shields each specialize in the four damage types?
Example:
Ancient Plate Shield:
+30 Normal Defense (15 Units)
Stun Immunity
Ironmight Plate Shield:
+30 Piercing Defense
Freeze Immunity
Volcanic Plate Shield
+30 Elemental Defense
Fire Immunity
Obsidian Plate Shield (WIP Name)
+30 Shadow Defense
Poison Immunity
Now as you can see, they'll all specialize in their own attributes, providing high amounts of defense along with one status immunity. It seems weird compared to the Armors, but with how Shields work, its even weirder that Volcanic pretty much outclasses everything. If they're too overpowered defensive wise, I do suggest decreasing their HP by a set amount.
If you make Plate Shields specilised in damage types it'll render every other shield redundant defensively. Only reason Ironmight isn't often used is because Barbarous Thorn Shield is more popular due to its Sword Damage Bonus.