I’ll be casting a 3” cube in polyester resin. You mention the difficulty of calculating the correct amount of hardener for added layers. Wouldn’t this problem be eliminated by letting each layer completely harden before adding the next layer? It worked fine with the epoxy casting I did. From the side you can faintly see the striations of layers but I don’t consider this to be a negative.
Yes, you could totally approach it this way. When you cast layers in quick succession, the heat is additive, so you add less hardener in subsequent pours (a polyester resin thing). As long as you let the resin fully cure (and cool), you can cast the next layer like you did the previous layer.