PlayerController.cs

public class PlayerController : MonoBehaviour {
    [SerializeField] private Rigidbody _rb;
    [SerializeField] private float _speed = 5;
    [SerializeField] private float _turnSpeed = 360;
    private Vector3 _input;

    private void Update() {
        GatherInput();
        Look();
    }

    private void FixedUpdate() {
        Move();
    }

    private void GatherInput() {
        _input = new Vector3(Input.GetAxisRaw("Horizontal"), 0, Input.GetAxisRaw("Vertical"));
    }

    private void Look() {
        if (_input == Vector3.zero) return;

        var rot = Quaternion.LookRotation(_input.ToIso(), Vector3.up);
        transform.rotation = Quaternion.RotateTowards(transform.rotation, rot, _turnSpeed * Time.deltaTime);
    }

    private void Move() {
        _rb.MovePosition(transform.position + transform.forward * _input.normalized.magnitude * _speed * Time.deltaTime);
    }
}

public static class Helpers 
{
    private static Matrix4x4 _isoMatrix = Matrix4x4.Rotate(Quaternion.Euler(0, 45, 0));
    public static Vector3 ToIso(this Vector3 input) => _isoMatrix.MultiplyPoint3x4(input);
}

with this code, it's impossible to control the magnitude of the movement with a joystick, is it possible to fix this?

with this code, it's impossible to control the magnitude of the movement with a joystick, is it possible to fix this?

You can normalize the vector, so if its 0, the magnitude is 0, otherwise is 1. Done that, you can simply play with the speed variable to have an "on/off" movement or add some acceleration time based.