Then make suggestions. As is it spawns magma (which I deem a positive side effect, because its the only magma-source kobolds have) and runs misery, doubling all negative thoughts.
The magma source workshop allows the filling of magma tankers, so it is not the only source of magma.
Flat out destroying a siege is very powerful and difficult to balance. However, I do have an idea, what if it just delayed a siege? Playing into kobolds penchant for sneakiness and deception, a reaction to trick the siegers into leaving, for a time. As far as implementation goes, as reaction that runs a normal slayrace, without the magma, but also infects the worker with a delayed force siege command. A random delay 2-3 years (or maybe less) with a limited resource as a reagent, gems maybe, would prevent it from being abused. You could try to delay multiple sieges, but the cost would get extreme.
Some intermediate reactions would also be useful; something that just crippled the invaders instead of outright killing them. The normal slayrace works by removing their blood; can you add things into their blood? You could do all kinds of stuff by introducing poisons into the invaders, or a portion of the invaders.
Warlock loot would need another use, perhaps a much needed improvement to kobold's magic system. Something that isn't just stuff ripped off from the orcs.
In general, for a theft based race, they are not as wealth based as I would like.
Not related to the above. All but three of the melee weapons kobolds can use are two-handed for them. Of those three, one is a shovel; one is a knuckle duster, a poor weapon that they cannot make; and the third is a pointy stick. They need more one-handed melee weapons.
They also need some more late game challenges, like a way to HFS.