I should see some nodes to grasp this, so I may have a look myself. I always make the greys first, then use them to displace. So, "thinking of that half-way point as the displacement (Y)" is not how I would do it, or see it, but again, I may not grasp your method.
You also tried using a bias and gain scaler after the smooth steps? These are handy to have better control over once established soft corners.
Matt was just ahead of my latest reply, but that is what I meant as well. Soft clamping, so to speak.