I am not saying safety is not VERY important.
I am just saying a heartbeat monitor is guaranteed not the way to implement it.
It is not the normal way to monitor safety and is plain inaccurate.
Set too low and the excitement of trying it shuts it off immediately, set too high and its useless.
FWIW, You should have a LARGE relay on the main power to the motors and in case the safety system is triggered, it will kill all motor power even in the event of the relay or mosfet controlling the motors short.