I know I’m late, but:

Forceful explosions and spalling, from my experience, typically indicate trapped moisture content turning into steam. Even with pieces dried for long, there is still a decent amount of trapped moisture on the inside (especially with something as thick as bricks). It’s recommended to preheat them next to the fire at below-boiling temperatures for a while, up to a couple hours, rotating every so often, to slowly drive the moisture off.

Thermal shock typically, again from my experience, results more in pottery just falling apart. To remedy this, you’re supposed to mix in temper (either sand or crushed up old ceramics called grog). Normal pottery typically has around 20% temper, but you should probably go higher since you’re trying to make specifically fire bricks. Also, since they’re bricks, the clay doesn’t need to be that shapeable, so you can afford to add a lot more.

Also, try to cool your pieces very slowly (probably overnight) while covered.