Relying on the caravans for your metal never goes well. They just don't bring enough to matter (Seriously, 4 bars of iron. Really!?) I know that caravans are supposed to be improved sometime in the future, but that doesn't matter now, does it? Probably best to use cage traps on goblins and strip them for metal unless you feel like abandoning that site. Or you can somehow bring magma up to the surface and use that as an effective, if somewhat crude, weapon. Or if you're in a somewhat adventurous mood, go find the adamantine spires and try your luck at those. You can bet that there are at least two or three of those on any embark.
it takes time. my last caravan from the mountainhome brought me 5-6 pages worth of bars, not counting smeltable crafts. just keep increasing priority of metal bars from the trade agreement with the liaison, and buying all their stock in those items every year. eventually, they will bring useful shipments.
also, do not neglect the human caravan. i buy all of thier too-big armor and immediately smelt it down.
But by the time they start sending enough, you're up to your eyeballs in...imported goblinware that you possibly won't need any from the caravans. Its not that they won't send any, it's just that when year 2/3 rolls around and the caravans have given you a total of 10 iron bars from ores/bars/melting, with the sieges probably already on their way assuming you haven't had one already...eh...what's the point then? You don't get what you need before you need it, and you can't exactly get it the moment you need it either, but oh boy, you can get some AFTER you need it....whoopie?
I guess what I'm saying is yes, I agree that it takes time. However in the long term the caravan is worthless for metal imports because your enemies will take care of that. They'll give you all you will ever need (unless you want something like platinum or aluminum). The only time the caravan REALLY shows its worth in importing metal is in the beginning, especially if you are in a metal poor region, but they don't really bring enough to be useful even then. Then the ambushes/sieges start and you just lol at the caravan.
my luck is such that goblins dont even seem to use bronze, much less iron. humans have bronze, but no iron. goblins have supplied me with much copper, but copper is not quite useful for armor or most weapons. that, and before i set my whole dog system up, and had the manpower to wear what copper and silver i had for weapons, it was better for me to just hole up till siegers got bored and went home.
on the other hand, trading with the dwarves has gotten me to 2 full squads decked out in steel, and most of the rest have at least bronze. sure, it takes a little while, but if you are in the situation i am in, where only dwarves can supply my metal needs, it pays.
typically, i use my cloth industry to boost the value of the clothes and leather the gobbos bring me, selling these for metal. the copper and silver the goblins bring is negligible, as i have both of those in relative abundance. their main value is saving a few of my bars for bolts, and giving me foreign weapons, like whips.
it was year 2 or so my first siege, year 6 or so before i had the manpower, training, and equipment to beat back a siege. now at year 8-9, the dwarven caravan only brings useful items (was sliding to this tendency since year 5), and even the humans mainly bring what i need, nowadays.
i suppose that you're right, goblins provide more metal than caravans, but sometimes worldgen is quirky, and it's not always useful metals the goblins bring.