You could open easily removing the 2 screws, remove the sd card and follow youtube instructions. When it’s open unplug the buttons connector. If the tablet stop rebooting is problem of the buttons if not I don’t know. The buttons could be easily bought on ebay. If the tablet stays switched off you could switch on with a small screwdriver touching in two pins at the back where you unplugged the buttons flex cable. You could also try how reacts when you charge and so on to discard battery, charging port…