Mac OS Big Sur | kernal_task usage is 500% when connected to external monitor
This may officially seem like the strangest answer ever, but have you tried using the ThunderBolt ports on the right hand side of the MacBook Pro?
As per this blog post, the thermal sensor in the MacBook Pro is located unhealthily close to the left hand side ThunderBolt ports, which will heat up and trip thermal throttling (the kernel_task spike you see), thus seriously hurting your user experience.