@mateuszga,
In a situation like this, you're better off managing the client off of the portal itself through another software deployment options (SCCM as an example). This way you can target specific collections however you want, without the limitations of what the firewall presents. This isn't the best solution honestly, and I've seen SCCM and similar solutions cause a few issues during the upgrade process.
If you choose to use the above method, I'd honestly split it across portals if you can get away with it. Have one portal for clients that can actually still receive updates directly through GlobalProtect (supported devices), and then have another one for everything else. You can direct the endpoints to the proper portal through Group Policy, but this allows you to actually keep using "normal" upgrade methods for the supported endpoints.
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