How to smooth out movement in a 3D space?

Hello Unity Community!

I am currently working on a 3D space combat game (chicken invaders styled). I've been working on my ship controller, using code like transform.Translate(-Time.deltaTime*speed,0,0); to move my object.

However, it seemed very jerky, so I tried using Vector3.SmoothDamp() instead. Here's how the code looks

 using UnityEngine;
 using System.Collections;
 using System;
 using System.Collections.Generic;
 
 public class ShipController : MonoBehaviour {
     public int speed = 20;
     public float smoothTime ;
     private Vector3 velocity = Vector3.zero;
     Vector3 targetPosition;
     
     private void Start()
     {
         smoothTime = (100/speed);
         targetPosition = new Vector3(-1000,0,0);
     }
     
     void Update() {
 
         //CLAMPING
         Vector3 pos = Camera.main.WorldToViewportPoint (transform.position);
         pos.x = Mathf.Clamp(pos.x, 0.09f, 0.91f);
         pos.y = Mathf.Clamp(pos.y, 0.15f, 0.9f);
         transform.position = Camera.main.ViewportToWorldPoint(pos);
 
         if(Input.GetKey(KeyCode.A))
         {
             transform.position = Vector3.SmoothDamp(this.transform.position, targetPosition, ref velocity, smoothTime);
         }
     }
 }

It works well, smooth, responsive and flexible. It has only one problem: since I'm using targetPosition = new Vector3(-1000,0,0); , my object translates to the left, but also towards the center. I want my object to move only to the left, and not hover towards the center, regardless of its current position.

Or, if someone could suggest and exemplify a different method of implementing this movement, it would be most welcomed.

Thank you for your time!