How do you go about easily producing reliable intricate dies and casts with novel designs?

That’s a tough scale to nail for one offs with one shot

I know precast is all the rage but surely there are better materials like metal or wood for particularly ornate design elements that don’t waste as much CO2 on potentially miscast half tonne experiments

Unless my intuition is wrong and cement casting is reliable and foolproof at any scale/desired fidelity?

You can do DIY mold forms using sand as a base and plastic sheeting or silicone in a box. This is most often going to be for precast stuff you mortar onto a flat surface elsewhere but you can create some very fun curved surfaces in place as well.

As for other materials, I love wood but concrete wins for water/fire resistance and metal is expensive.