I want to turn off cellular data for most System Services

My cell service provider imposes fees if I use more than 25 MB of cellular data per month. So I use cellular data very sparingly. Unless there's an emergency, I keep cellular data turned off for all apps except Phone.

But that strategy isn't working, because the massive amount of cellular data used by System Services is not under my control. For example, Push Notifications used 77.9 MB. That alone has blown out my 25 MB-per-month limit. (And that's a crazy amount of data. I get very few push notifications. They are tiny snippets of text, with no graphics. If coded efficiently, I think they would use less than 1 kilobyte per month.)

There is no switch to turn off cellular data for Push Notifications, or any other of the System Services. And that is truly a bug in iOS which is costing me money every month. I could easily live within my 25 MB-per-month limit, if it weren't for all the cellular data wasted by System Services.

I want to turn off cellular data for all System Services except Find My (because Find My doesn't use much data, and it's essential – if it were turned off and I lost my iPhone, the iPhone would be well and truly lost).

Does anyone know of an app that would allow me to do this?