One central point?
I wasn't viewing this network in those terms. Not with the way the UFP is scattered across the nearspaces of the Arm, as you've observed (and I agree). It's a distributed multi-hub network. Not unlike the Net itself.
And the ECS would not be the only component of the UFP's merchant marine. Every member world would have a service akin to ECS, and many of the older colonies not yet graduated to full-member status would be building such as well.
Liking how this discussion's going. It's productive. Stirs the brainpower.