Cutting the top off a hemlock?
I would stay away from anything fast growing. That leaves you with the same problem you have now where the bottom branches die off. And they are dying off from lack of light, which isn't always a bad thing. (Weeds grow slower.)
Hacking the tops off the trees will make them look bad imo. I would either cut them down and start over or leave them as is. But anything that you find that grows well in low light likely won't give you the privacy you want.
It sounds like you might be best with a fence.