You can do this with the IFTTT shortcuts already - look for “Edit Shortcuts” under the three dots in the upper right corner of the iOS or Android app. You can have large number of cameras do a wide variety of things based on an equally wide variety of events. And it’s very simple to set up.
I have almost the exact situation that you describe - when our cheapo Harbor Freight entrance alarm sounds off at one entrance, it triggers a sound event on the nearest camera. Which then activates the “Automate” feature of the shortcut. Which then tells several other cameras in different locations ( on completely different LANs and different internet connections ) to record events ( “upload a short video to the cloud” ) at the same time. So the entrance, parking lot, street, store interior, back lot, etc are all immediately filmed from different angles at the same time. Works great!
The one thing you cannot do is specify a ‘group’ of cameras to do something when creating the shortcut. Each camera must be individually added to the shortcut and edited one at a time. Which gives you very granular control, and is not big hassle since it only happens at setup and most people aren’t setting up a huge number of shortcut actions.
The IFTTT shortcuts are also very handy to turn off/on notifications from a number of cameras at once so your phone doesn’t drive you crazy while you are working in front of a camera group for a long time unloading a truck, etc.
I haven’t worked on a way to do geofencing yet, though…