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,

Let it discharge completely then fully charge it and then reset.

That happens when you need you to visit your service provider or either give them a call or visit their website.

Have you tried upgrading your HDD to SSD and also upgrading the ram and if that is not the case please make sure you check your charger and cable cords and also check the inside for possible board contacts with casing based on a motherboard scratch or cable contacts.

Try to use a VGA cable directly to a monitor or connect a HDMI to a TV either ways should work there is also SSD removal option which is easier.

I personally use a heating pad to conduct heat evenly with 75* C for about 8 to 10 mins based on the room temp and if you use 60*C let it sit for around 15 mins or more.

As per my experience I can show you an easy way to almost bring any dead device back to life, I have encountered various types of handheld technology and have been experimenting on them but so far found the best solution that doesn’t need anyone to be a technician or disassemble anything, all you have to do is just grab an old tooth brush and dip the tip of the bristles in 70% isopropyl alcohol and brush the charging port of your device and clean the inside of it, there is always an 80% chance of the charging port leads to be dirty or the charging port stuffed with dirt and grimes please do so and let me know if that works.