I'd say that the forearm to racket angle stays the same with a neutral wrist when placing the reference points of the hand on any given bevel. The strings face different angles as the racket is rotated in the grip but the forearm to racket angle does not change. To change the forearm to racket angle you need to put one reference point of the hand on a different bevel.
For example, a 'Strong Continental' grip changes the forearm to racket angle by moving one reference of the hand 1/2 bevel or 1 bevel. For example, index knuckle on bevel #2, area near pinkie centered on bevel 1 1/2 or 1.
To change the angle of the strings relative to the hand rotate the racket in the grip. To change the angle between the forearm and racket move one reference point of the hand. Try it with a racket.
The grip fixes the racket to the hand but the hand is moved by the wrist joint a short distance away. I don't know how the forearm to racket angle interacts with wrist joint motions so this applies to a neutral wrist position.
Also, the 'fat pad' reference of the hand is usually not touched by the racket handle in high level serve grips. The butt of the racket usually is placed around the little finger area. My opinion based on high speed videos.