I have a few project ideas, and I thought of reusing the paper from various shop catalogs that I receive in my physical mailbox. I’d like to make it stiffer, something more like cardboard. I read somewhere online that you could use corn starch for this, mixed with water. Would it work? Do you have better ideas?
Your best bet would be PVA (Poly vinyl alcohol), which is a water soluble plastic and usually the base of wood glue or water based paper glue.
Dilute it in warm water and soak your paper in it.
Starch is too stiff and crumbly for that use case.