Electronics are going to be much more reliable
than any heath-robinson contraption you could reasonably build.
Two very simple and extremely reliable options:
1) A clock. After a given time, fire the rocket. Calculate the likely maximum flight time to altitude and add a bit, such that it will fire after balloon burst.
2a Strain gauge joining truss to balloon not via the parachute. When strain drops to near-zero for longer than 5 sec or so, fire the rocket.
2b) If you want to wait for the parachute to have opened and thus stabilised the truss, use two strain gauges - one on the parachute cord, one on the balloon cord.
When parachute cord has higher strain than balloon, fire.
2 a/b can be done entirely with discrete solid-state electronics (wheatstone bridge, op-amp, monostable, mosfet) no software or moving parts. (Avoid electrolytic caps)
Use two identical strain gauges in both cases to compensate for temp - both should respond the same.
Can easily test this in the freezer as pressure has no effect, only temperature.