What Android apps are most helpful in preventing mobile data leaks in the background by apps as well as controlling individual apps use of mobile data?
On the Moto e4. It has a setting. Called data saver.
Have you enabled developer mode (click build number until popup says you're a developer)? There's an option in developer options to close every activity as soon as you leave it. Lots of apps continue to run in the background after you move on to another activity. This will stop it.
That might help a bit.
I looked at Google play for you.
Search for data saver. A bunch of 4 star apps show up