So I was bored and put your original numbers/recipe into Bru'n Water.
First, you don't need that much epsom salt to get Mg. There is plenty in grain that will promote yeast health. And, I think this is part of the harshness as this adds to bitterness.
Second, your sulfate to chloride ratio is actually 2.7:1, which is quite high if you ask me. Especially a very hop forward beer with a hop know to increase harshness.
Third, this doesn't account for the "added minerals" to your Gerber pure so the minerals would be higher, possibly substantially higher.
Try my advice above. I think you'll run into better beer. Remember, your water salts are like cooking. Use salt and pepper for the main flavor (gypsum and Calcium chloride) and use the other spices (baking soda, epsom salt, pure salt) to "season" the water.