"Can I go even thicker and still cold form it by hand?"
It depends how good you are with a hammer. The problem is hammering will work harden and, more importantly, fatigue the steel and you will start to see stress cracks, especially at the edges. If you have access to a propane torch (or whatever witch's brew is replacing propane) you can anneal the steel at red heat. Red heat also greatly eases hammer shaping. I like like working 12 gauge hot rolled steel cold.
I have never tried working wrought iron cold.