You have to buy a new phone.

I really mean it. I know that is not what you want to hear, but that’s the reality.

Every version of Android below Android P allows apps to access the camera from the background without consent:

https://www.xda-developers.com/android-p-background-apps-cam...

I don’t like disposable hardware as much as the next person but I’m absolutely not going to go without basic security to die on that hill.