Trade took me a little bit to figure out. Here are some tips.
Tips to Trade
You can indeed ship stuff 2 ways, but I’d encourage you not to use the same spaces for different goods. As long as each space is used for 1 good every leg of the journey, the cargo will never spill over into other slots. So with the coal and iron trade you mentioned, if you’re using a schooner, then the first slot could carry coal, and the second could carry the iron back. If you try to have both slots pick up coal on the first leg and then iron on the second, it’ll work for a little while but sooner or later it will clog up your routes. Always split 1 good per slot and never mix.
I like to use regional hubs. I have a route that uses cargo ships for example that transports wood, brick, steel, windows, concrete, and aluminum between my old world capital, crown falls, and manola. I have it set to pick up goods only from the islands that actually produce it, so if crown falls doesn’t produce any steel, I have it drop off only and not pick up from there. (If I have a direct feeder route setup, such as from another island in the cape that produces steel and ships it directly to crown falls, I count that as producing it) So I always pick up aluminum from Manola, but never steel as it can’t get produced there. It only drops steel off, and aluminum only gets dropped off at the other 2 hubs.
From those hubs, I set up local routes. Again using construction goods, I have ships pick them up from those local hubs and transfer them in a cycle from one island to the next in that region. For these routes, I have them setup as balancer routes. Meaning I set a minimum required amount for each of the goods handled for these routes, then I have them drop off and then pick up from each of these islands. So they pick up from the hub and then basically start to distribute them to every other island which ensures that whenever I want to build something that requires steel, I don’t have to find a ship to do it and then wait for it to arrive. I know steel has already been delivered, or another batch will be delivered again soon. If the island already has enough, it just picks it back up and moves on to the next island that might need it.
This system I use depends heavily on feeder routes. Basically what you described with 1 resource, 1 way routes that move the resources from the island that produces it to the hub for that region so that I can ship it to another hub which will disseminate it along to the other islands there.
For goods that aren’t required in large numbers, I’ll have it shipped by a schooner or clipper where I give it 1 or 2 slots and other goods occupying the others. If the demand for the good becomes too much for just a single slot or 2, I’ll create a dedicated route for it and add more and more ships to it until it can maintain constant supply. Of course you need to make sure that the harbors at both ends are upgraded before just adding more and more ships to it as I’ve found that to be the bigger limiter for me more often than not. Fewer ships can often handle the task, as long as they’re able to grab a full load every time they visit which means you need local storage to be high enough for that to be the case. If you only have 150 max storage on your island, with 4 clippers on a route moving goods from there, and yet failing to transfer enough goods to where they’re supposed to be, adding another clipper will be a very poor option cost efficiency wise. Upgrading the harbor will go a much bigger way to addressing the issue.