Can a GPL v3 backend be used in an iOS app without exposing all the code?

Question

Licenses for software and other works

Good afternoon.

We are developing an iOS application where all interfaces are based on MIT/Apache/BSD components while the back end is entirely GPL v3.

All these licenses are compatible, so there is no problem with this.
The main problem is, can we not open source our client code and apply the terms of the GPL to the entire application?

The backend and frontend are an integral part of the application (their functionality cannot be separated), so the entire application must be licensed under the GPL, based on the standard license terms, which is completely unacceptable to us.

Our idea for avoiding open code is that we distribute only part of iOS to the end client.
The backend is not distributed, it only runs on our servers, not iPhones/iPads, so we treat this part as for your own use ;)
So the question is how fragile is this position in case of litigation and what is the current practice?

Thanks in advance for comments.