Unity: how to force rotation after rotating towards a direction? - unity3d

Question

After asking here for a solution, which worked, my sphere "walks" to each point of a list. Now, I'm trying to "force" a rotation around the x axis, to fake the ball is "rolling". I came up with the following code, but it doesn't work (it makes a progressive acceleration of the rotation on the x-axis, which I both dont want and... dont understand).
Create a new project, add a sphere and add this small script. Then add 2 points in the points list. This script does nothing but moving the object (the sphere) each points of the list, and make a simple rotation around the x-axis.
Simple rotation around the x-axis is not realistic (it's working as long as the sphere doesn't change its direction, but in this case, it should rotate on x + on z, but I dont know how to do this too...) so if you have a more realistic idea its even more welcome.
using UnityEngine;
public class MoveAlongPath : MonoBehaviour
{
public Vector3[] points;
public float speed = 1.0f;
public float rotateSpeed = 200.0f;
public float minDistance = Vector3.kEpsilon;
private SphereCollider _c;
[SerializeField] private Vector3 _posStart;
[SerializeField] private Vector3 _posEnd;
[SerializeField] private int _currentIndex;
[SerializeField] private Rigidbody _rigidbody;
[SerializeField] private Vector3 _direction;
[SerializeField] private Vector3 _directionNormalized;
[SerializeField] private float _xRotation;
private void NextPoint()
{
_posStart = points[_currentIndex];
transform.position = _posStart;
_currentIndex = ++_currentIndex % points.Length;
_posEnd = points[_currentIndex];
_direction = _posEnd - _posStart;
_directionNormalized = _direction.normalized;
}
private void Start()
{
_rigidbody = GetComponent<Rigidbody>();
NextPoint();
}
private void FixedUpdate()
{
_xRotation = (_xRotation + -10 * Time.fixedDeltaTime) % 360;
Quaternion q = Quaternion.LookRotation(
_directionNormalized, Vector3.up
);
transform.rotation = Quaternion.RotateTowards(
transform.rotation, q, rotateSpeed * Time.fixedDeltaTime
) * Quaternion.AngleAxis(_xRotation, Vector3.left);
Vector3 step = speed * Time.fixedDeltaTime * _directionNormalized;
if (step.magnitude > _direction.magnitude) {
step = _direction; // don't overshoot
}
Vector3 newPos = transform.position + step;
_rigidbody.MovePosition(newPos);
if (Vector3.Distance(_posEnd, transform.position) < minDistance) {
/* close enough -> next point: */
NextPoint();
}
}
}