If you change the parent it should be fixed.
"parent": "item/handheld",
You have set the attack value to maximum usage and the default attack speed of a diamond pickaxe is -2.8F i guess.
super(Tier, Tier.getMaxUses(), 0, new Properties().group(Group));
protected PickaxeItem(IItemTier tier, int attackDamageIn, float attackSpeedIn, Item.Properties builder){...}
I also would recommend you to create a toolmaterial enum which implements IItemTier for custom tool materials.