How to tilt 1st person camera's Z axis when moving left and right?

Hi everyone,

I'm still somewhat new to unity and was trying to create a 1st person controller. Basically, I wanted to tilt the camera's z axis whenever the player would be pressing A or D (similar to Quake's camera tilt). I've encountered an issue with my mouse script, which is locking the camera's z axis.

Here's the very basic tilting script:

 [Header("Peaking")]
     public Transform PeakLeft;
     public Transform PeakRight;
     public Transform IdlePeak;
     public float lerpTime = 0.05f;
 
 
     void Update()
     {
         if (Input.GetKey(KeyCode.A))
         {
             transform.position = Vector3.Lerp(transform.position, PeakLeft.position, lerpTime);
             transform.rotation = Quaternion.Lerp(transform.rotation, PeakLeft.rotation, lerpTime);
         }
         else
         {
             transform.position = Vector3.Lerp(transform.position, IdlePeak.position, lerpTime);
             transform.rotation = Quaternion.Lerp(transform.rotation, IdlePeak.rotation, lerpTime);
         }
 
         if (Input.GetKey(KeyCode.D))
         {
             transform.position = Vector3.Lerp(transform.position, PeakRight.position, lerpTime);
             transform.rotation = Quaternion.Lerp(transform.rotation, PeakRight.rotation, lerpTime);
         }
         else
         {
             transform.position = Vector3.Lerp(transform.position, IdlePeak.position, lerpTime);
             transform.rotation = Quaternion.Lerp(transform.rotation, IdlePeak.rotation, lerpTime);
         }
 
     }

And here's my the mouse script (it's inside of the general character controller script):

 private void MouseLook()
     {
         rotationX -= Input.GetAxis("Mouse Y") * lookSpeedY;
         rotationX = Mathf.Clamp(rotationX, -upperLook, lowerLook);
 
         playerCamera.transform.localRotation = Quaternion.Euler(rotationX, 0, 0);
         transform.rotation *= Quaternion.Euler(0, Input.GetAxis("Mouse X") * lookSpeedX, 0);
 
     }

Whenever I disable the Quaternion.Euler lines in MouseLook(), the tilt script works perfectly fine, so I assumed it's because the z axis is getting locked at 0. I'm not entirely sure what should I do. Is there a way I could modify my mouse script to make the tilting work? Thanks.