I did a quick search about using glue to make quills more robust yet keeping them flexible.....and found almost nothing on this matter.
For many years now, I have been applying rubber cement (the stuff to repair inner tubes) to my turkey quills, crow quills, etc.
I buy the glue in the small metal container that also sports a brush to apply the glue. I apply the glue to the inside portion of the quill until the quill is well soaked.
I then let each quill dry separately buy sticking the quill in a block of styrofoam. Unfortunately, the glue never really dries out because if you accidentally touch the quills together, they will immediately stick to each other. However, for hoppers and many other flies, quill sections can be applied in such a way that they do not come in contact with one another.
The nice thing about using rubber cement is the fact that the quill stays flexible but the fibers of each quill section will not separate.