I am making a 2.5D game where the player can only move up, down, right or left; no X-Axis

I have the controls set-up like this:

using UnityEngine;

public class Movement : MonoBehaviour
{
    public float speed = 8;
    private Vector3 scale = new Vector3(5, 5, 5);
    private Rigidbody rb;


    void Start()
    {
        rb = GetComponent<Rigidbody>();
    }

    void Update()
    {
        transform.localScale = scale;

        if (Input.GetKey(KeyCode.S))
        {
            transform.localScale = new Vector3(5, 2, 5);
        }
    }

    void FixedUpdate()
    {
        float mH = -Input.GetAxis("Horizontal");

        rb.velocity = new Vector3(0, rb.velocity.y, mH * speed);

    }
}

the problem is that the character should turn when it is heading in the opposite direction, i could just flip it with the rotation or set the Z scale negative, but that would make it turn istantly, i want to have it turn in the span of a few frames so it doesn't seem unnatural. How can i proceed to implement something like that? Is it even possible in unity?

examples of what