nei If the goal is to prevent such apps from ever connecting to the real internet, how safe would it be to only grant the network permission when the phone is connected to the dash cam Wi-Fi with mobile data staying off?
The network permission isn't designed to stop data exfiltration. Apps can exfiltrate data without ever needing the network permission.
nei The app could even be disabled when not used.
Disabling apps doesn't stop them from storing data when you use them.
In practice, your idea "might" work for a dash cam app that's not trying every trick in the book, but that "might" depends on you actioning those precautions without fault.
I personally do not trust myself to juggle settings like that, and would sooner just not use the app if possible, get a dash cam that doesn't require an app, or use a secondary device that's effectively air gapped for this purpose.