trying to make a smooth rotation script

Hello, I'm kind of new in learning Unity scripting and currently i'm writing a script where by pressing the Space key together with A or D key would do some sort of a "dash" movement by rotating faster. In the gif below I show the basic A and D movement without dashing movement. What i've done is that i've put an empty with the script and an object parented to the rotating empty.

What I wanted to do is that whenever the player wanted to go faster on a direction, it would "dash" on the left by pressing A+Space and on the right by pressing D+Space. I kind of managed to do this thing with this code (where rotSpeed is the rotation speed and dashAmount is the speed added to make the "dash"):

 transform.Rotate(dashAmount + rotSpeed * Time.deltaTime, 0f, 0f);

Of course, this worked but the cube would just teleport instead of "dashing" smoothly to the left or right. Then i found out about Quaternion.Lerp but the code i wrote doesn't really work correctly (destinationRot is the final rotation and dashSpeed is set to 0.8f)

         if ((Input.GetKey(KeyCode.Space)) && (Input.GetKey(KeyCode.D)))
         {
 
                 Quaternion destinationRot = Quaternion.Euler(transform.rotation.x + dashAmount, 0f, 0f);
                 transform.rotation = Quaternion.Lerp(transform.rotation, destinationRot, Time.deltaTime * dashSpeed);
 
 
         }

How can I fix this?