I think that Thranduil as tank is forced. He is father of Legolas and in time of Hobbit the best warrior in Middle-earth. He was created before Botfa and because of that the game feeling while playing with him is not so good. Instead of agile mass slayer we have slow tank. I think that make him mass slayer with unit support aspect (because of king) is great idea. And i always thought that Grimbeorn would be perfect tank - he is old, strong and beornings were tortured by orcs so they can sustain a lot damage. Switching their roles would solve the problem with roles in Lorien.
Also I am not sure about that arcane shield, Thranduil is not magician.