To add some realism, instead of simply having 2 planets and freighters going between them, why not have the fact that a trade route is a serie of planets having a spaceport, and linked all together in a trade system. You would be limited to the number of "trade steps" and there would be a simple equation to show $ influences, including these parameters:
The range between each step of the route
The population of every planet on the trade route. So making a detour to your trade route to add a huge metropolis would be good.
Multi-Empire route. If you are the only ones friend with the Altarian, they could allow one or 2 of their planet to be part of your route (maybe Terminus?) and that means more customers!
That way, you could have parts of the chain attacked and broken, but the whole trade route could still survive (until too much attacks) but on the short-term, you loose $$$ 'cause your chain is broken.