As regards protecting a phone, I did give that some thought. Like most folk I have some old phones lying around. It could make sense to use one of those on the dash connected via wifi to your "good" phone - all smartphones can do that. Then if the "decoy" is snatched you have the good one to ring the Gardai, locate the "decoy", etc... The downside is that you wouldn't be able to direct dial mytaxi clients through the app unless you have an active SIM in each phone. Either way it's probably worth hiding an old phone connected to a charger in the boot, even without a SIM it can make emergency calls.
Of course you can go the camera route which, in most cases, will deter runners, etc. However, that could go the other way if you encounter one of those who are intent on robbing you as the camera may prompt them to destroy all possible evidence - the obvious would be to burn the car when they're done.