Switching grips is not that difficult, and I mean for ROS against decent, not 120+ mph serves.
You feel with your OFF hand, on the throat of your racket. You hold it the same every time, the grip that gives you the correct topspin grip.
Racket hand is LOOSE, barely gripping the racket. Squeeze a bit if the serve goes to your forehand side.
Takeback and shoulder turn is led by your off hand. That is all the backswing you need against fast serves.
Don't lean back, stay on the balls of your feet.