the best way to fix this is just give dks something around 10 to 15% dmg aura buff atleast but they wont do it cuz dks are fine in raid and threat in raid is not a problem and as you may understand blizzard balance everything around raiding so no hope for this issue my friend.
the dk tierset was good in dmg and then they nerf it to 3sec internal coldown which is not legit garbage and useless no hype for tierset now unless they atleast fix its 2set so it gives us 2 or 3% strength also they should remove heartstrike at the 2set and make it all the abilitys then maybe you have a chance to have fun with dk