I don't think you have to hit harder OR more spin.
What you gotta do is place the ball better into the sideline corners, deep.
Most 4.0's can hit hard enough when they have time to set.
All 4.0's cannot set early enough on every ball, and often miss the shots by going wide a foot or two, instead of getting it IN by a foot or two.
Work on placement.
McEnroe and Connors never hit hard, they just placed it really well near the sidelines, over and over again.
Lacking good placement, then you need to hit harder.