Well I'm just throwing this out there, as I don't know if this could be possible, but could you make it so that dramatically shifting the Beam target drains Ki? That way it'd act as a method of limiting how much you shifted it without actually restricting it.
Right now you just charge it and let'er rip, regardless of what's happening, and there's no consequence for it. If the enemy moves or comes after you, you'd now spend extra Ki to make up for using it at the wrong time.
This could allow other changes to Beam attacks as well, such as the speed it charges/travels and initial Ki costs. If you can't move the Ki attack around as much, you can afford it these bonuses to balance it out.