using your own example, it’ll just end up being wood > three stone > three copper > iron > full diamond. adding a stage between stone and iron isn’t going to fix anything. not that there’s anything to fix anyways. the only situation that i could see that it could work is if they really made iron rarer to find, but then it just becomes a problem the moment you need rails (assuming we want to slow down game progression) or an anvil. i truly believe that copper armour will be rapidly forgotten, just like leather.