I think Grotesques are pretty over-costed for what you get in return, same with both the cronus and the talos really
.
Covens in general this edition are pretty poor because most of their units points to damage ratio is off, and they are slooooow.
I think you should look to taking wracks over Grotesques if your dead set on covens. Your list also has no way to kill tanks from range that will be a huge issue for you.