How to limit kernel_tasks CPU usage?
Kernel_task is not just one process, but a representation of all the kernel's input and output. High k_t usage can be a symptom of problems, rather than a cause in itself that needs to be corrected.

Causes could include third-party kexts, Spotlight indexing, or other background processes. One of its functions is to use up CPU as a means of restricting other processes, in order to prevent overheating.

I had that model of MBP and did not observe it becoming 'unusable' ever, so it may be some issue relating to your installed software or preferences.

There is little need or benefit to re-installing the OS on a regular basis.