Disclaimer: I am not an Electrical Engineer…
There is some basic protection already in the circuit. The diode on the power in line, the diode on the power regulator output, the mosfet on the Arduino 5v in line, the optcouplers on the flash and camera triggers. Better spec’d optocouplers could be used though.
An additional layer of protection could be added by having optocouplers before the TIP120s but I believe this is only really required if you also wanted to add separate power supplies for each valve. Using one power supply for everything adds convenience but also means nothing is truly isolated.
My main concerns when developing the device were protecting the camera and the flashes, and not trying to pull too much current from the Arduino at any one time.
If you want to look in to this further, look at the guides on the Rugged Arduino site (https://www.rugged-circuits.com/10-ways-to-destroy-an-arduino/).There is another good site that I can’t find/remember. If I do I will add a link.