[Suicide Junkie]

You could make the asteroids into slow moving fighters... They'd cruise along mostly in straight lines, until they need to turn to avoid a collision.
Not too bad, except that they would either turn to avoid ramming you, or come in to attack.

I'm not sure how you could cause them to split...
You could have all of them carrying fighter bays, and have the large asteroids launch smaller asteroids, which launch really small asteroids, and all would possibly drop containers of "metal"

The problems would be the launching of the babies only when its about to die, and the attack/avoid movement routines.
I suppose you could give them very low turn rates relative to their speed, but then they'd crash into each other.