While I cannot answer your question
(for sure) about isolation of every app, but as far as I understand it on Android the apps are isolated - sandboxed (even without Graphene), and GrapheneOS adds the option to block Native Code Debugging, but:
I use contacts with network:
-not willing to mess around with syncing
I use Google Messages with network:
-RCS Chats requirements
I use Google Dialer with network:
-Filter Spam calls
I use Google Clock with only Notifications:
-Completes the picture (all 4 apps at the bottom same color etc)
I use Gboard as Keyboard with out any permissions:
-just used to it 
Somewhere along the privacy journey you will start weighing Privacy / Inconvenience / Convenience and make decisions.
These are mine (for now)
First and foremost if it’s tooo inconvenient and complicated: Not for me…
So my threat model (that regards my age of 45) is security - usability - privacy.
Don’t have the nerves and energy to mess around much (maby youngsters do)
For the saved time I’d rather go hiking 
Edit (forgot direct answer lol): So if you don’t use them with network permission, I personally don’t see any problem