Change material on multiple objects when nav mesh agent reaches waypoint

Hello, I am very new to Unity and know I am doing things very unorthodox, so I hope this makes sense! I created a bunch of different walls, all with the same material. A nav mesh agent navigates along a series of waypoints. All I want to do is change the material or shader of every wall every time the agent reaches certain waypoints. I have tried the following a number of different ways: public class mazeRend : MonoBehaviour {

     private MeshRenderer _mazeRend;
     private Shader shader1;
     private Shader shader2;
     private Shader shader3;
     private Shader shader4;
 
     
     // Start is called before the first frame update
     void Start()
         
     {
         _mazeRend = GetComponentInChildren<MeshRenderer>();
         shader1 = Shader.Find("Unlit/Texture");
         shader2 = Shader.Find("Mobile/Diffuse");
         shader3 = Shader.Find("Toon/Basic");
         shader4 = Shader.Find("Standard");
 
         _mazeRend.sharedMaterial.shader = shader1;
 
     }
 
     // Update is called once per frame
     void Update()
     {
 
     }
     private void OnTriggerEnter(Collider other)
     {
         if (other.CompareTag("char_1"))
         {
             _mazeRend.sharedMaterial.shader = shader2;
         }
     }
 }
 

I have also tried indexing Material[] with an int = 0, and updating with Material[int++]. I have tried linking the above script with my characterController script, such as

if(characterController._currentTarget == waypoints[4]){_mazeRend.sharedMaterial.shader = shader2;}

When using colliders, I get a NullReference error and when linking scripts, nothing happens.

Also, all 4 shaders are attached to each wall.

Any help would be so so so much appreciated! Thank you anyone that offer any solutions/advice!!