Some item errors?
It seems you confused the block rating on it with defense - in other words, this item has 0% avoidance and is not useful for a max mitigation set.
Also, the shield choice for a "threat" set is baffling to me. As I understand it,
[Kaz'rogal's Hardened Heart] is the best TPS shield, due to its hit. I also would venture a guess that the additional stamina and defense on it outweighs the armor mitigation and block value of the
[Vengeful Gladiator's Shield Wall]. Or you could socket it for even more hit.
Item theorycrafting: You made the same mistake I did in my initial item theorycrafting: Do not neglect 30% more shield block value from talents, as well as 10% more armor. For instance, a ring with 44 block value * 1.3 = 57 effective block value. Small perhaps, but notable. A ring with 400 armor really has 440 effective armor, and it goes even higher if you factor in inspiration.
Final note: I value hit and expertise most for a maximum threat set, especially for trash tanking. I have a DST and while the sustained TPS output is good, consistent "snap" threat at the start of a pull or fight is typically the most important.