Not sure if you have tried, but iTunes allows one to do a Device Firmware Update (DFU) of any Apple device with an IPSW file (basically ios firmware) practically making them a brand new device.
Although it is a bit of an overkill, because technically you would be overwriting the iOS firmware with a stock one. It does help. I had to use it on my aging iPad 2, a couple of times and haven't face any issues thus far.