Giuseppe Cassibba

Google FCM and Apple APN are used, based on what I can read in https://rocket.chat/docs/administrator-guides/notifications/push-notifi…, only for mobile devices when you decide to use Rocket.Chat official App. In that page they explain also how to avoid push notification via these components. In my honest opinion, if you want completely secure your connection from these notification, you could use only web interface. Thnak you for sharing your thinking!

I think that Arduino hardly can do this job. Arduino is a microcontroller, while Raspberry PI is a micro computer. Arduino runs based on program instructions. Raspberry PI runs based on a complete OS.
I love Arduino too, and I wrote a comparison between the 2 boards in https://peppe8o.com/arduino-vs-raspberry-pi/.