Perhaps to give you an idea of the weight of steel to HDPE (this is a little lighter than polycarbonate)
I replaced 5mm steel top plate on Saint with 20mm HDPE and it was about half the weight too!
the only thing that you could replace the polycarbonate with, would be HDPE as this dose not crack like polycarbonate.
But you may like to mix and match the armour, think what actually needs protecting and if it doesn't need armour
don't waste weight protecting it.