What if non-core cities could build food (or give food maybe as a percentage of excess food), or build gold or hammers to core settlements?
I think your system there is too complicated but the general thought is right. Civ 3 had caravans that could provide hammers when disassembled in other cities.
Finish a wonder in 1 turn? Easy, just build 10 caravans and send them to your city right when the new tech comes out...
Which is NOT what I want. So they need to decay if not used, and they need to be national units, like missionaries.
But let's say that for the cost of x hammers, you can build
product shipments that are worth half x hammers. (with guilds: enabled, cost 100 worth 50. With logistics: 150 to 75. Machine Tools: 200 to 100. Assembly line: 250 cost for 150 worth. Final stages could be with robotics/automation.)
For each turn these shipments are underway, they lose 5 hammers of their worth. Using them to build a wonder makes them only half their worth.
So they could both represent colonies supporting core as well as the other way around.
Also,
food shipments. You have to build a shipment for say, 50 hammers (surplus food goes right in), and then fill it with 50 food from the cities granaries. Off it goes to feed your starving cities elsewhere. I never understood why we "open our foodstores" to help other civs, but our own cities are left starving. And Rome grew big on grain shipments from Egypt. That said, maybe enable a 20 food version for ye olde classical times and the 50 food version with refridgeration.
And for what it's worth,
trade shipments could also be nice. Enabled with economics, you build a 100 hammer unit that conducts a trade mission like a 1/10th great merchant, or something. (is it just me? I am never using GMs for trade missions if I can use them for stock exchanges)
This could be implemented in UHV goals of trade orientated civs as well. The Tamils come to mind, who could get their unique unit with that ability.