Let's turn the Smith machine into a lying leg press machine. You'll need to set the pins or safety catches here so that the bar is resting on them at the bottom. It will need to be high enough for you to wedge yourself underneath the bar, but low enough so that you're getting a decent range of motion. No three-inch rep stuff.
Smith Machine Leg Press
Try this: Start with a hard set of 10, take some weight off, and immediately do another hard set of 10. Repeat that one more time and find your safe space.