Thermite sounds a bit excessive for a typical home or office. How about baking in an oven or roasting on a grill? Is modern flash memory fragile enough to be destroyed irreversibly at 230C/450F?
They have to be rated to survive those temperatures for short periods of time during reflow soldering.
Apparently flash memory is rated to last a minimum of 10 hours baking at 125c, or 360 hours at 85c. The decay is exponential, so in theory 30-60min at 230c should do a lot more damage.
But I'm not sure you should rely on it, that 10 hours @ 125c number is for when the bit-errors exceed the capabilities of the ECC. The data will be partially recoverable for much longer. Or you might have some flash which does a lot better than the minimum spec.
How about microwave? Powerful charges at every exposed terminal looks like a nice way to fry some chips. Maybe submerge the whole PCB in a bowl of salt water to prevent sparks flying everywhere...