Bronze tools/weapons building should require (Copper Smelter and Tin Smelter) or (Trading Post and Copper Ingots and Tin Ingots). No need to include the other options Copper Smelter and Trading Post and Tin Ingots as the Copper Smelter means you have copper ingots etc.
In the bronze age the ingots were "hides", ie shaped like a cow hide; that required 4 men to lift. We know they were traded from shipwrecks of the era. Egypt took many hides as tribute. BTW they also took glass ingots (thick discs) as tribute also.
Shouldn't we just make a new building, the Bronze Smelter, and have that building require Copper AND Tin? And make it produce Bronze? That would seem to me the logical solution.