So I understand that you intend to merge the bronze and iron smith chains and I definitely think this is a good idea. I would restructure the smithing chains into "practical smiths", as such, and artisan smiths. While iron is definitely the key functional metal for the Continental Celts at this point, bronze was still substantially used for art and bronze helmets were not uncommon. For this reason your two branches could consist of:
- Blacksmith (minor weapons buff, agriculture & industry buff due to tools manufacture)-> Ironsmith (better weapons buff, minor armour buff for medium/heavies due to ringmail, less of an ag & ind buff as more time is spent on weapons & armour)->Swordsmith/Armourer (great weapons buff and a decent armour buff for heavies due to chainmail but no ag & ind. buff, a master swordsmith does not make tools!)
vs.
- Bronzeworker (minor armour buff, small culture buff, small happiness boost: the Celts loved their art, crafts)-> Goldsmith (culture buff, happiness boost, more crafts, morale boost due to the manufacture of torcs, which Celtic warriors believed granted divine protection)-> Artisan Smiths (basically more of the previous)
NOTE: I mentioned ring mail and crafts. Ring mail is admittedly purely speculative, it seems like a likely step towards chainmail. When referring to crafts, I would love to see a 'crafts' trade resource added to reflect the export of artwork from many cultures, such as from the Etruscans to the early La Tene (although of course Etruscans are gone by our period, I assume they will be removed?). Obviously some cultures were bigger exporteers than others and this would need to be reflected in the amount of goods generated by the buildings. Amphorae, for example, were widely exported and so a Roman craft chain should produce a great deal more 'crafts' than the Celtic equvilent, who largely just traded these goods with eachother, but did so greatly.
Other Artisans branches could include:
-Craftsmen: reflecting the wonderful panoply of woodworkers, bone-sculptors, masons and other assorted craftsmen who enriched Celtic culture, in game they could replace the 'carpenters' and provide a mix of small buffs to construction, happiness, health(?), culture and craft exports.