Stiffness isn’t great-- if it has to move a bone with high stiffness, it will.
If you only want two bones to move with the IK, then only have two bones in the IK hierarchy. Here’s an example where I use a non-deforming 2-bone IK and I have 4 deforming bones parented to the bones in the IK chain:
Notice the hierarchy in the outliner.
Typically, people are doing what you want to implement twist bones. What you’ll notice is that the structure pictured above does what you’re explicitly asking for, but you might actually want something a little bit different than that. Here’s a typical twist bone setup:
Here, the first bone in the chain isn’t parented to the bone in the IK chain, but to the previous bone (upper thigh parented to hips, upper calf parented to lower thigh), but damped tracks the appropriate twist bone, which is parented to the IK chain.