Should the wrist be solid or loose during external rotation?
When it comes to pitching, IMO nothing should be stiff. Being stiff causes stress and stress causes injuries.
“Hold the ball like an egg, because it is one.” -Crash Davis
Keep your whole body loose, relax, don’t over think your motion.
An in case someone crushes the ball off you remember “I held the ball like an egg and he scrambled it.”