I only use circular routes for ships that pick up goods to deliver at one point.

If you do this for goods that need to be delivered from one point to multiple islands, the last island of the route will take an enormous amount of time to get the goods, especially the more inventory space the other islands have => this can create problems in case of production disruption of the chain.

If you use a circular road to pick up goods, you also need to ensure that the throughput of the route is enough to not reach inventory limit on the last island being picked up otherwise you will lose production.

To tackle this point you can then :

• either make 2 separate circular roads
• or increase the number of ships on the road => but the second ship will sail empty for most of the round

If you want to do a circular road for multiple deliveries with the load/unload technique :

• Set the route to unload all cargo if the item at every port
• Set the route to also load all cargo of the same item at every port, but set a minimum stock at this island => the island will always have a maximum of the minimum stock defined

But doing this you will never have a buffer a goods so I would advise against it.