The only problem I need to solve now is that the entire circuit exploded after firing the first shot. ⚡💥 I believe it was the same mistake and failure mode - the thyristor was wired as a high-side switch this time deliberately for convenience. As soon as the thyristor is turned on, the gate voltage rises to 500 volts.

I thought the transistor was protected by the new diode I added in series, but no, it's again a n00b mistake. The base-collector junction of a BJT is also a diode, and you can't connect any diodes in series without an RC snubber to balance the voltage. Without balancing, series diodes simply break down one after another. #electronics