When System is blocked, it forces the phone to reboot. I am using NoRoot Firewall, it has also a log, so it shows allowed/blocked connections.
I have Google Search, Calendar and stuff like that blocked, it desperately tries to call home several times a minute, lol.
On Android this can be circumstances with stuff like download manager.
That's why GrapheneOS use own network permission which fix that.
Other apps like Netguard have the same problem. There is also no need to talk about Afwall due to insecurity and security theater.
I don't see any advantage in crippling down your system that way without full control and real security. A firewall doesn't protect against OS or firmware security problems.
Sure it can be used as best practices but nothing more.