Have you tried using groups and train interrupts? In my set up I'm able to have generic cargo trains that go to cargo pick up stations when they're enabled and from that inventory they're able to automatically go to that resource's cargo drop off station. That cargo train is part of a "cargo" group ...

Couldn't agree more. The 2.1 changes are great but it makes space buffer stations very difficult to achieve.

For example if I want to keep a supply of uranium fuel cells at a space station over Gleba then the station will request all the cells I keep on Gleba before a resupply ship can make it ...