So it’s not UL listed?
Do you need the 15V input to torch the capacitors and run the relays? Could I use a 5V USB ESP32 dev board instead, and just put a boost circuit or MOSFET on the relay load side for the explosion circuit when it needs to go off, or are there other components that need the higher voltage?
You could, but from my measurements the capacitor tends to pull ~1A before it bursts, so you'd be running up against the 3A limit of USB. At voltages below 12V, they don't seem to go off reliably.
Have you tried a backwards polarity tantalum capacitor? I've seen those go off just on their normal voltage when installed backwards. Sometimes immediately and other times with significant delay.
Eye Projectile and Eye Flash protection along with hearing protection are required.