Modding can do heaps. You need a world regen. Many changes will work though if a save itself has the raws modded (each save game propagates it's own copy of the raws as they were when that world was create), the changes I outline are NOT of these type - so new world only.
The civ definitions are in the entity_default.txt file . There's a list of the stuff they carry. Keep a backup vanilla file, but you can delete lines for e.g. toys / instrucments from the human trade goods. Which would get you more % of all other types of stuff they bring. You could also ban their reaction for bronze i think, which would make other metals more common.
open DF\raw\objects\entity_default.txt, search for "human", then page down, here's the type of tag you'd remove to stop humans creating/trading a type of thing :-
[TOY:ITEM_TOY_PUZZLEBOX]
[TOOL:ITEM_TOOL_KNIFE_CARVING]
Removing too many of these types of things could have some sort of adverse affect, but I've removed all useless-seeming tools, toys and instruments without any seeming problems.
For the bronze reactions, try removing the lines :-
[PERMITTED_REACTION:BRONZE_MAKING]
[PERMITTED_REACTION:BRONZE_MAKING2]
One uses bars and the other uses ores. You could maybe add these reactions to humans if you like, they don't seem to have them :-
[PERMITTED_REACTION:BISMUTH_BRONZE_MAKING]
[PERMITTED_REACTION:PIG_IRON_MAKING]
[PERMITTED_REACTION:STEEL_MAKING]
I've had the odd crash or wierd stuff from playing too much with the Dwarf civ entity though, so there's limits in what works (can't tell you the EXACT limit though, so play around)