A few comments.
There’s a practical upper limit of around 30 nodes per channel, and a hard upper limit in the code of 250 (according to this post Theoretical Mesh Stats? ), so you might want to keep that in mind and compare it to how many head of stock you’d need to virtually fence in. (I think you can use multiple channels simultaneously, so if you need 100 head of cattle, you could maybe use 3 or 4 channels with disconnected meshes of 25-33 devices each? )
There’s a plan to implement a way to read/write gpio pins - but it’s not in the code yet. Discussion here: Extension of the functionality of devices Meshtastic
If I were approaching this problem, I’d wait for (or help out implementing) that remote-gpio-module, and then take the entire meshtastic hardware/software project as my “transport layer”, hand some minimal hardware off the gpio pins to drive the buzzer/e-collars/sensors (there’s about a dozen gpio pins broken out you could use), and do all the geo fencing and command/control stuff on a RaspberryPi or a laptop at “home base” using the Python APIs here: https://meshtastic.github.io/Meshtastic-python/meshtastic/index.html
Good luck!