// Store parallel arrays for our movement parameters.
private float[] movementSpeeds;    // Randomized speed per object.
private int[] nextWaypointIndices; // Destination per object.

private void Start()
{
    // Changing terminology to be more standard.
    // - a "prefab" is a source asset you want to copy
    // - an "instance" is a copy of it that's been spawned in the scene
    instancesToMove = GameObject.FindGameObjectsWithTag("FrameLine");

    movementSpeeds = new float[instancesToMove.Length];
    nextWaypointIndices = new int[instancesToMove.Length];

    for(int i = 0; i < instancesToMove.Length; i++)
         movementSpeeds[i] = Random.Range(minSpeed, maxSpeed);
}

private void WaypointsAI()
{
    // We want to iterate over the instances, not over the waypoints.
    for (int i = 0; i < instancesToMove.Length; i++)
    {
        // Check to see if we reached the last waypoint.
        // If so, we can stop here & skip ahead to the next instance.
        if(nextWaypointIndices[i] >= waypoints.Length)
           continue;
        // You can also prune instances that are done to save processing.


        // Look up the waypoint we want to move toward next.
        Vector3 waypoint = waypoints[nextWaypointIndices[i]].position;

        // Read this instance's speed,
        // to compute how far it should move in this frame.
        float travel = movementSpeeds[i] * Time.deltaTime;

        // Apply the motion.
        Vector3 position = instancesToMove[i].transform.position;
        position = Vector3.MoveTowards(position, waypoint, travel);
        instancesToMove[i].transform.position = position;

        // If we're within a configurable tolerance of the
        // destination waypoint, set our destination
        // to the next waypoint after it.
        if((position - waypoint).sqrMagnitude < 0.1f)
            nextWaypointIndices[i]++;
    }
}

// Adding selectable behaviour at the end of the path,
// as requested from the comments.
public enum PathCompleteBehaviour {
    Stop,
    Loop,
    Reverse
}
public PathCompleteBehaviour completeBehaviour;

// Store parallel arrays for our movement parameters.
private float[] movementSpeeds;    // Randomized speed per object.
private int[] nextWaypointIndices; // Destination per object.

private void Start()
{
    // Changing terminology to be more standard.
    // - a "prefab" is a source asset you want to copy
    // - an "instance" is a copy of it that's been spawned in the scene
    instancesToMove = GameObject.FindGameObjectsWithTag("FrameLine");

    movementSpeeds = new float[instancesToMove.Length];
    nextWaypointIndices = new int[instancesToMove.Length];

    for(int i = 0; i < instancesToMove.Length; i++)
         movementSpeeds[i] = Random.Range(minSpeed, maxSpeed);
}

private void WaypointsAI()
{
    // We want to iterate over the instances, not over the waypoints.
    for (int i = 0; i < instancesToMove.Length; i++)
    {
        // Check to see if we reached the last waypoint.
        // If so, stop / loop / turn around as needed.
        int waypointIndex = nextWaypointIndices[i];
        if(waypointIndex < 0 || waypointIndex >= waypoints.Length) {               
           switch(completeBehaviour) {
               case PathCompleteBehaviour.Stop: 
                   // Leave it where it is and skip to the next instance.
                   continue;
               case PathCompleteBehaviour.Loop:
                   // Wrap front to back / back to front.
                   waypointIndex = (waypointIndex + waypoints.Length) % waypoints.Length;
                   break;
               case PathCompleteBehaviour.Reverse:
                   // Bounce back to the point before last.
                   waypointIndex += (movementSpeeds[i] > 0f) ? -2 : 2;
                   // Reverse direction.
                   movementSpeeds[i] *= -1f;
                   break;
           }
           nextWaypointIndices[i] = waypointIndex;
        }
        

        // Look up the waypoint we want to move toward next.
        Vector3 waypoint = waypoints[waypointIndex].position;

        // Read this instance's speed,
        // to compute how far it should move in this frame.
        float travel = Mathf.Abs(movementSpeeds[i]) * Time.deltaTime;

        // Apply the motion.
        Vector3 position = instancesToMove[i].transform.position;
        position = Vector3.MoveTowards(position, waypoint, travel);
        instancesToMove[i].transform.position = position;

        // If we're within a configurable tolerance of the
        // destination waypoint, set our destination
        // to the next waypoint after it.
        if((position - waypoint).sqrMagnitude < 0.1f) {
            nextWaypointIndices[i] += (movementSpeeds[i] > 0f) ? 1 : -1;
        }
    }
}