SOLVED I took my 2018 i9 (32GB) to the Genius Bar. Internally it was really badly blocked up with dust. Air couldn't get in, and air couldn't get out. Blowing all the dust out has made my machine 'breath' properly again. The kernel_task issue disappeared immediately after doing this.

Has anyone tried connecting to your external display via HDMI only? (I don't have a dongle to try it)