More details: https://www.howtogeek.com/337719/how-to-tame-your-chromecast...
What the hell is a idle chromecast doing everyday? That's absurd. Is it outbound WAN traffic or just local network traffic.
I did install a firewall/data logging app on a spare Amazon Fire HD8 and found it downloaded several hundred megabytes of advertisements over a week's time despite sitting in a drawer unused. That was an easy fix.
Another glaring data hog was an Amazon FireTV stick which was set to the highest quality.
Overall it's been an eye opening experience to see how many devices consume bandwidth with no regard for optimization or giving users control.
But only get a UDM pro if you don't need link agger or the ability to control multiple networks from the same controller (i wanted to throw a USG at my parents but can't join it to the UDM pro because.. reasons). it has some annoying limitations (IDS/IPS cannot keep up with my internet anymore) that if i knew about i'd have gotten a USG-pro and a cloudkey.