Power-down sequencing with a "hard" power switch (stepper driver)
I have simple recommendation to make the energy stored in your C1 capacitor more useful. Stop using 7812 and 7805 for U1 and U2. Use a switcher instead that can provide the needed voltages over a broader range. Even better if those could be buck/boost style combo mode switchers.

Alternately you could look at a better PS1 solution that supplies all three rails or multiple PS1's where PS1A provides the 24V and PS1B provides the 12V and 5V or an additional PS1C that provides the 5V.