If you just want to exclude App Store distribution, you don't have to modify the license. Just use the GPLv3, because the App Store terms of service are incompatible with the GPLv3. It should not be possible to have your GPLv3 app in the App Store.
Article from the FSF: https://www.fsf.org/blogs/licensing/more-about-the-app-store-gpl-enforcement
Each time you redistribute the Program (or any work based on the Program), the recipient automatically receives a license from the original licensor to copy, distribute or modify the Program subject to these terms and conditions. You may not impose any further restrictions on the recipients' exercise of the rights granted herein.
(Emphasis added.) This last sentence is a crucial part of the strong copyleft in the GPL and AGPL: it prevents distributors from using separate legal agreements, like Terms of Service or NDAs, to take away the freedoms that the license is supposed to grant. This is the license condition that Apple is violating when it distributes GPL-covered software through the App Store.
But there some minor loopholes:
Ad Hoc - Limited to 100 Devices. Devices must be explicitly added to a provision.
Enterprise - No device limit, devices do not need to explicitly added to provisions. In effect, these builds will run on any device; the caveat, you are not legally allowed to distribute these builds to anyone outside your company.
Maybe this still fills your needs.