Question After transform.Rotate() the Object forwarding is not changed, just reset originally. Help.

Code (CSharp):

1.                
2. void Update()
3. {
4.                 float rotationDirection = _input.Player.Move.ReadValue<float>();
5.                 transform.Rotate(Vector3.up * Time.deltaTime * 30 * rotationDirection)
6. }
7.  
8. void FixedUpdate()    
9. {        
10.                  if (rb != null)        
11.                  {            
12.                                  rb.velocity = transform.forward * speed * Time.deltaTime;        
13.                   }    
14. }
15. ;

This code keeps the player going straight after turning.

Code (CSharp):

1.  
2.  
3. void Update()
4. {
5.                 float rotationDirection = _input.Player.Move.ReadValue<float>();
6.                 transform.Rotate(Vector3.up * Time.deltaTime * 30 * rotationDirection);
7.                 var smooth = 2.0f;
8.                 var tiltAngle = 30.0f;
9.  
10.                 if (rotationDirection > 0)
11.                 {
12.                     rotationDirection -= tiltAngle;
13.                 }
14.                 else if (rotationDirection < 0)
15.                 {
16.                     rotationDirection += tiltAngle;
17.                 }
18.                 else
19.                     rotationDirection += 0;
20.  
21.                 var target = Quaternion.Euler(0, 0, rotationDirection);.
22.                 transform.rotation = Quaternion.Slerp(transform.rotation, target, Time.deltaTime * smooth);
23. }

However, if the player's tilt is added like this, the direction cannot be maintained after rotation and is immediately restored to the initial direction using 'FixedUpdate()'.

Should I maintain forwarding after transform.Rotate() ?, please.