Why don't you go three wire?
Have power, ground and signal. Make signal a half-duplex RS485 system.
What I might envision is that you "tell" each microcontroller what the delay is.
Have an ARM, FIRE, ABORT, ACK and NAK signals.
So, you would preload each timer with the delay and get a confirm back from the timer.
While this might require a laptop you could create a processor just for the load sequence and have 3 buttons, LOAD, ARM, Start, Abort and a few lights. Each delay would then be relative to the start pulse and not each other.
But 32 nodes is a problem and the distance is a problem.
https://www.rs485.com/pfaq.html
Or, basically modulate the power supply rail with 2 tones for the arm sequence. Use a DTMF (2 tones) decoder to detect the start sequence. This means power electronics at each node and you have to manually set each timer.