Even more ideas
Its also feasible to make a small "blob" of low melting point gallium/indium alloy which melts at a relatively low temperature.
This is an order of magnitude more sensitive than the loop of nylon but has a breaking strain in the multiple kilos.
Also two electrically separate resistors, thyristors and batteries connected to the "base" means if one independent firing circuit breaks during ascent then the other should fire.
Use optical LM567 based triggering so no wire is needed, with the correct frequency being needed for it to fire.
As a second backup, have an altitude based trip which severs the connection to the balloon if the internal pressure exceeds a set limit, with three sets of rotors and a tilt sensor to keep the payload upright during descent so the rocket can fire.