Can't get lerp working with toggleable crouch

So I found a script in this forum that lets the player crouch and lerps between standing/crouching size. The problem I'm having is that I'm wanting a toggleable crouch state, but modifying the script to include that seems to break the lerp. I can still toggle between crouching/standing and I can see the states changing in the inspector, along with the speed change working fine.

Here is my script:

 using System.Collections;
 using System.Collections.Generic;
 using UnityEngine;
 
 public class PlayerMovement : MonoBehaviour
 {
     public CharacterController controller;
 
     public float speed = 6f;
     public float sprintSpeed = 10f;
     public float crouchSpeed = 3f;
     public float jumpHeight = 1.5f;
     public float gravity = -20f;
     
     public float startHeight;
     
     public bool isCrouched;
 
     float maxJumpsNum = 1;
     float currentJumpNum = 0;
 
     public Transform groundCheck;
     public float groundDistance = 0.4f;
     public LayerMask groundMask;
 
     Vector3 velocity;
     public bool isGrounded;
 
     void Start()
     {
         controller = GetComponent<CharacterController>();
         startHeight = controller.height;
     }
 
     void Update()
     {
         Movement();
         Crouch();
         Jump();
         WorldHandler();
     }
 
     void Movement()
     {
         //w/s/a/d input.
         float x = Input.GetAxisRaw("Horizontal");
         float z = Input.GetAxisRaw("Vertical");
 
         //normalizes the movement vector by making its magnitude, on any of its 
         //axis, extend to the maximum length of 1. stops faster 
         //diagonal movement in this case.
         Vector3 forward = transform.forward;
         Vector3 right = transform.right;
         forward.Normalize();
         right.Normalize();
 
         //a vector3 for the movement input - normalized.
         Vector3 move = (right * x + forward * z).normalized;
 
         
 
         if (Input.GetKey(KeyCode.LeftShift))
         {
             //makes the player sprint.
             controller.Move(move * sprintSpeed * Time.deltaTime);
         }
 
         else
         {
             //makes the player move.
             controller.Move(move * speed * Time.deltaTime);
         }
     }
 
     void Crouch()
     {
         float newHeight = startHeight;
 
         if (Input.GetKeyDown(KeyCode.C))
         {
             if (!isCrouched)
             {
                 newHeight = 0.5f * startHeight;
                 controller.height = Mathf.Lerp(controller.height, newHeight, 5.0f * Time.deltaTime);
                 speed = crouchSpeed;
                 isCrouched = true;
 
             }
 
             else
             {
                 newHeight = 1f * startHeight;
                 speed = 7f;
                 isCrouched = false;
             }
         }
 
         float lastHeight = controller.height;
 
         
 
 
         Vector3 position = transform.position;
         position.y += (controller.height - lastHeight) * 0.5f;
 
     }
 
     void Jump()
     {
         //makes the player jump (mathf square root (h * -v * g)).
         if (Input.GetKeyDown(KeyCode.Space) && isGrounded)
         {
             velocity.y = Mathf.Sqrt(jumpHeight * -2f * gravity);
             currentJumpNum = 0;
         }
         
         //makes the player jump again if the conditions are correct.
         else if (Input.GetKeyDown(KeyCode.Space) && currentJumpNum < maxJumpsNum)
         {
             velocity.y = Mathf.Sqrt(jumpHeight * -2f * gravity);
             currentJumpNum = 1;
         }
 
     }
 
     void WorldHandler()
     {
         //projects a sphere from pre-defined position by [x] distance, looking for a ground layer collision.
         isGrounded = Physics.CheckSphere(groundCheck.position, groundDistance, groundMask);
 
         //applies a tiny amount of downwards velocity constantly to help with ground checking.
         if (isGrounded && velocity.y < 0)
         {
             velocity.y = -2f;
         }
 
         //applies gravity to the player.
         velocity.y += gravity * Time.deltaTime;
         controller.Move(velocity * Time.deltaTime);
     }
 }