Greetings everyone and by the way the USB-C on iPad 12.9 4th gen is replaceable and I had this issue I'm an apple repair tech so I deal with them all the time if there is a charging problem always try to clean the port with a tooth brush dumped in alcohol which is really effective and opening won't be necessary but if that won't solve the issue then you might have to open the screen and change the port with a lot of care not to break the flex face id cable which is located beside the front camera but mounted to the screen itself including the front speaker and two light sensors but in conclusion the USB-C is changeable on this model fortunately,