Now try convincing JimmyConnors to drop his bolo overhead stroke.
OP, the reason the straight elbow doesn't work, and you don't bend it after you start your foreward swing enough, is that like a dancer spinning, the arms IN help you cartwheel faster, while arms OUT, slows down your rotation.
A serve needs precision, like a catcher's throw.
It doesn't need sheer power, like an outfielder's throw.
Look at some short swing pitcher's, like BartoloColon, who barely bring the hand back, but still throw fastballs with accuracy in the low '90's.
Bend you elbow for trophy, usually 90 degrees.