As far as I've seen, if you're talking Hardox, 5mm is ambitious. My bot has a 4mm scoop and that is completely rigid and feels thicker than you'd think. I think most people have 2-4mm steel or Hardox armour, otherwise it gets very heavy and quite expensive. HDPE you want about 20mm for the sides, 10-ish on top, but since you're doing a wedge you might want a bit thicker, I'm not sure.