Move Gameobject towards/away from position based on the proximity of 2 other objects

OK, so I have 4 cubes in a scene. I'm trying to achieve the following:

• Player moves cubeA towards cubeB.

• At a specific proximity of cubeA to cubeB, move cubeC towards cubeD.

• If the player moves cubeA away from cubeB, cubeC returns back to it's original position.

• The proximity of cubeA to cubeB can be reached in any vector3 position. However, cubeC and cubeD should only move towards/away from each other along the same axis.

So far, I'm using MoveTowards for the first part.

 public Transform CubeB;
 public float proximity = 5.0F;
 bool ProximityReached; 
 public Transform CubeC;
 public Transform CubeD;
 private float AdaptsqrLen;
 
 void Update() {
 
 Vector3 offset = CubeB.position - transform.position;
 float sqrLen = offset.sqrMagnitude;
 if (sqrLen < proximity * proximity){
     ProximityReached = false;
 }
 else {
     ProximityReached = true;
 }
     
 AdaptsqrLen = sqrLen -5;
 CubeC.transform.position = Vector3.MoveTowards(CubeD.position, CubeC.transform.position, AdaptsqrLen);
 }

This method works for the first part, without adapting sqrLen. However, when I subtract from the sqrLen to let MoveTowards returns negative values, CubeC position flickers between two positions on the same axis, either side of CubeD - probably because I'm not declaring what direction to move it away in.

Any thoughts how I can adapt this to achieve the objective?

Thanks!