That's pretty much it. It would being huge peace of mind to know mic & camera are turned off after a call, similar to what GOS already does to Wi-Fi and Bluetooth.
Feature request: turn off Mic & Camera automatically
Good idea . But if implemented, it should not only work for phone calls but also for all apps where you could activate mic and cam like camera apps, messenger apps (voice message/video calls) etc.
It is recommended to leave those permissions enabled globally and grant one time access to apps. See https://github.com/GrapheneOS/os-issue-tracker/issues/5132 for more details from one of the GrapheneOS developers.