I'm not sure, but maybe disabling background activity might help. Also, why are you using such abusive apps?
Are you certain you are not using such apps? If so, please tell me how I can easily determine which apps that request access to my microphone are not selling my data :-)
What you suggest seems like an obvious feature though. Unfortunately I found if you can't do something on iOS, you really can't do it unless you jailbreak.
That said disabling background access does really quit most (all?) app activity some time after you switch apps. It's not a perfect mitigation, but such is the cost of buying into closed ecosystems :(
Edit: The other answers suggest something else is probably at play, so disregard this.