Not as crazy as it sounds!
No guessing required. Just look at how professional motors of this size have been started quickly for many years. Heat flux + pressure threshold. Due to the large core, getting it up to the pressure threshold quickly is a challenge we don't typically have in regular grain geometries. You cannot rely on self-pressurization from the initial start of the top grain of the motor. Putting enough heat flux into the top grain to fill the motor could damage the grain.
More practical details would probably have to be in the research forum. A rough calculation gives about 100g of 'good' igniter mass, distributed to light as much of the pyrogen surface area as quickly as possible. But, lightly contained to not puke out the nozzle before doing its job.
Not to violate rules but a nearby "thermite" expert might be of assistance eh? I had a "non-physical" scare with the stuff and don't have need for it so haven't revisited it. (Spell that a loud scary bang of an open test conducted at a proper distance) Had a physical "scare" too:
No I didn't hold something to my ear I shouldn't. Stupidhead was too close to a conventional igniter test using a laptop to activate the igniter through an Arts II and no BP or BP like substance was involved. The igniter "popped" up
caught, and hit the ear. At least I had a chance to look away and my arm pushed the wire away and I managed not to burn my arm.
Now my cardinal rule (and it should be everyone's too) is first time test or first multiples of tests of anything that involves "fire" needs to ALWAYS be done at the appropriate distance. The hotter the fire, the farther away.
And sheesh, this happened a few years after my thermite scare so Stupidhead should'a known better. Kurt