So. A thought occurs. I've been poking through the threads pertaining to resource distribution systems, markets, and allocations et-cetera. The thought is... Admittedly a bit complex.
First: Rather than having trade caravans manage ALL of the distribution, do what I've seen done in games such as Sierra Studio's Pharaoh and Ceaser city-builders. Add workers to the Barn/Warehouse/resource storage buildings. As an example, 3. Now. Said workers don't have to go out and collect resources from worksites. Their job is *purely* delivery. Whether it be to another resource center within range or to a nearby building site that needs materials delivered. Add a tie-in to the donkeys system to increase overall load capacity of each worker. Their job is purely to utilize and manage the *rest* part of this thought.
Second: A per-building demand sheet. Have a barn/warehouse in a residential district? Demand a steady supply of food. Add modifiers for when the storage should call out for more resources when the current stock falls below a percentage threshold starting at like say... 50%. Want to keep enough resources in the residential to be able to build one house at any given time? Add demands for lumber, stone, etc. If the stock gets *too* low on the current supply, the workers are forced to divert from wherever else they might be delivering to in order to prevent the stock from hitting zero making it crucial to ensure the demand of the area doesn't get overwhelmed by the neighboring storage facilities ability to keep it supplied.
Third: Donkeys. Given the game only permits a given citizen to carry a single type of resource at a time, donkeys should be on hand to speed up/add variety to warehouse deliveries. That said: there should only be so many donkeys per actual worker. For realism sake, say 3 donkeys per worker. With 1 workers, that's 3 donkeys and four types of resources that can be delivered to any single location. At maximum: 3 workers, 9 donkeys, and a whopping 48 resource types that can be delivered *at a time,* making it easier to keep up with high-demand areas.
Fourth: Range. Much like the hunters lodge, foresters, and gathering huts, workers range shouldn't be unlimited or they might die in transit. Resource facilities should *only* deliver to other facilities of the same within range. Planning on building a sprawling city across an island but want split production into districts? You're gonna have to either use caravans, or stack barns to 'hop' resources along the routes to get to different parts of the island. Buyer beware: someone else might need those resources while they're in the barn so they may never make it to their destination. Unless that was your plan of course.
It should be noted that the facilities should *obviously* still be used by people when there's no workers assigned to it, otherwise towns would starve or freeze without a hectic amount of early micromanagement until you've got enough pop to sustain yourself.
Thanks for reading!