The basic system you describe for pilum is exactly what they always were - they had always been designed the bend or otherwise be unable to be thrown back.

There were a number of variations on the theme, and I'm not sure we know precisely when each was introduced.

For example one version had the metal shaft pinned with 2 pins - 1 metal, one wood - the wood pin would break and the head would pivot about the metal one, making the weapon useless until repaired.

There were also light and heavy pila, although precisely how often a mix was carried isn't known - it may be that teh 2 pila usually carried represent 1 of each.

Light pila usually had a socketed metal shaft fixed to the wooden section. Heavy pila seem to have had a large metal tang at the base of the metal shaft that was riveted to a large block of wood at the top of hte metal shaft. Heavy pila could be weighted further with lead.