How to strengthen the plastic thread for repeated attaching/detaching of a garden hose?
It's called "a very short hose". :) You can make your own with parts commonly available at hardware and big-box stores. 

Alternatively, install a simple valve, an elbow, or a backflow preventer:







Be aware that extending a plastic connection with a rigid part adds torque stress to that connection. Use caution to not lever on it.