QUick question regarding Billboard Script.

I think what you want is the billboard object to only rotate about the Y axis. I've been using this script on the Hololens. You specify either Free or Y axis rotation in the inspector. You may need to modify it for your own purposes. I hope this helps you.

 // Copyright (c) Microsoft Corporation. All rights reserved.
 // Licensed under the MIT License. See LICENSE in the project root for license information.
 
 using UnityEngine;
 
 namespace HoloToolkit.Unity
 {
     public enum PivotAxis
     {
         // Rotate about all axes.
         Free,
         // Rotate about an individual axis.
         Y
     }
 
     /// <summary>
     /// The Billboard class implements the behaviors needed to keep a GameObject oriented towards the user.
     /// </summary>
     public class Billboard : MonoBehaviour
     {
         /// <summary>
         /// The axis about which the object will rotate.
         /// </summary>
         [Tooltip("Specifies the axis about which the object will rotate.")]
         public PivotAxis PivotAxis = PivotAxis.Free;
 
         [Tooltip("Specifies the target we will orient to. If no Target is specified the main camera will be used.")]
         public Transform TargetTransform;
 
         private void OnEnable()
         {
             if (TargetTransform == null)
             {
                 TargetTransform = Camera.main.transform;
             }
 
             Update();
         }
 
         /// <summary>
         /// Keeps the object facing the camera.
         /// </summary>
         private void Update()
         {
             if (TargetTransform == null)
             {
                 return;
             }
 
             // Get a Vector that points from the target to the main camera.
             Vector3 directionToTarget = TargetTransform.position - transform.position;
 
             // Adjust for the pivot axis.
             switch (PivotAxis)
             {
                 case PivotAxis.Y:
                     directionToTarget.y = 0.0f;
                     break;
 
                 case PivotAxis.Free:
                 default:
                     // No changes needed.
                     break;
             }
 
             // If we are right next to the camera the rotation is undefined. 
             if (directionToTarget.sqrMagnitude < 0.001f)
             {
                 return;
             }
 
             // Calculate and apply the rotation required to reorient the object
             transform.rotation = Quaternion.LookRotation(-directionToTarget);
         }
     }
 }