I’m not aware of how 74HC595 typically fails but I’m thinking that, in a worst case scenario, if 24V gets from the triac gate to that pin, the resistor is not much of a help.
Maybe you have more experience with these circuits.
Have you considered using SSRs on the driver side?
AQY282SX from Panasonic looks a good pick and are quite cheap.
Would this be feasible?