If you’re comfortable pulling th iPad apart, then do so. There are a number of parts inside that are connected by flat cables into ZIF sockets; these sometimes work loose. I’ve also pulled apart iPads (not this particular model) and found a ZIF socket broken after a heavy-handed screen replacement. A loose connection can signal a reboot request by mistake.
Also check the battery is plugged in properly.
Apple don’t really want people poking around inside their gadgets; it’s not designed to be easy. if you’re not comfortable doing this (see the iFixit repair guides), then get a pro to look at it (or get yourself a new one).