The executive branch churns every 4 years, and is forced to churn at least every 8 years.

That only applies to political appointees. The rank and file are permanent employees (or were until a few weeks ago).

Anyways, not saying your idea couldn't work, only that it's not easily implemented and needs a lot of consideration to do well. It's a wholesale change to how we've governed ourselves for ~150 years. But, the idea of a permanent set of legislative experts has some appeal.

We haven't governed ourselves with an executive-branch-led bureaucracy for 150 years. It's been about 70-80 years total. This model started with FDR and was really expanded in the 60's and 70's. The US existed for most of its lifetime without a permanent bureaucracy. The system you are talking about as the one that has worked forever is much younger and much less stable than you think.