It seems to me that these older 9.7 inch models (2016) are pieces of crap.
I’ve never had one, but i can only assume a logic board or charging port issue.
Since i have done various Android, iPhone, and iPad repairs, i would buy a iCloud locked variant that is otherwise IDENTICAL to the broken iPad. At this point i would start swapping parts and testing it out.
Overall, i would just turn silent on and hope and pray it doesn’t vibrate.
Best of luck, Austin