Plugging anything into Thunderbolt inc bare cables causes kernel task overload
I've actually had the same issue forever with my mac. The same device as yours.

I took it in to get fixed just before my 3 year warranty expired and unfortunately I thought it was fixed, but it wasn't. (Very regrettable that I didn't notice it wasn't resolved till after the extra 90 day warranty had expired.) 

I honestly think it's a defect in the device, but have no idea how to fix it. I've tried many solutions over the years and just found your post as I'm restarting my search.

It is so frustrating to not be able to use the second monitor I have sitting here.

Not a solution, but one thing that helps a little with the kernel overload is running an actual fan across the surface of your computer.

If you've come up with anything, please do share.