Why are there so many unexpected shutdowns in the event log?

I do know that every so often I have to hold the power button down to shut off the computer because it gets stuck in the "Shutting down" screen

Because Windows uses a new boot way called fast startup mode, which is doing a logoff of the user and now hibernates the kernel. During boot it resume the kernel and brings you so faster to the logon screen.

But hibernation can sometimes cause issues (drivers or BIOS problems) and it looks like it doesn't work for you correctly.

Turn it off in the control panel.

and now Windows uses the old way to fully shutdown and this should fix your issue.