'''++@joef5d++'''
since you’ve already tried the calibration etc. it does stand to reason that this is a hardware fault. I would repalce the analog sticks and see if that will make a difference. Problem is going to be to tear it down, no manuals available so you rely on your skills. So when you do take it apart create a guide for th erest of us. Use this guide https://www.ifixit.com/Guide/new Then you have to find the right potentiometers as well…….