Delayed start with RC and MOSFET
Did I miss something?


Yes, the capacitor will remain partially charged once the switch is opened causing the circuit to activate significantly quicker the next time the switch is closed. This may or may not be problematic but, a solution is to ensure the switched supply properly collapses.


  I planned to add a 12V Zener at the M1 gate, will it be sufficient to
  discharge C1?


No, because the zener may not conduct much current significantly below its zener point and this may still leave a high enough voltage to short-cut the delay at start up.

What engineers usually do is put a reverse diode across R1 and provide an unambiguous discharge path on the switched supply: -



But it all depends how critical the timing must be.