This solved it for me also!

SOLVED!!! I had the same issue with my 2018 i9 32GB ram. None of the above resolved my issue. Ended up at my local Genius Bar yesterday. The machine was packed FULL OF DUST. We cleared all that out and now the machine can breath and cool itself properly again the kernel_task problem has gone away.

Same boat mate. I have an 2018 i9 with 32GB also, It's so frustrating, I want to throw it out of the window! I'm wondering if connecting to my Dell UHD display via HDMI would make any difference?