This repo has block lists for your Pi-hole/ emummc.txt that can help you set up a method to block all communication with Nintendo. This will stop any updates and reporting to Nintendo, but it will prevent use of the eShop and online games.
You can find a very well written guide to do this here
Feel free to submit a pull request if you find more hosts, and I will happily review and merge to this repo.
Note: Nintendo may add more hosts with time. The most sure way prevent any communication of your Switch with their servers is if you 'switch' on (hehe) Airplane mode when running CFW.