I'm sure you guys would prefer for me to actually be writing the book, so I'll be Supaa brief.
Empires put a great deal of energy into their expansion. It gets harder and harder to expand but, of course, Every linear expansion of the empire brings an exponential increase in the costs. It's just not clear what those costs are. The Solar Union is a 20th century-style soft empire. The colonies don't technically have their goods stolen; they sell them to the SU. But the SU hasn't allowed them full participation, in that they're straight up poorer. So the SU sets a "fair market price" and will only pay that — or at least "recommends" it to the corporation in charge of a particular expedition. On Quall, that corporation backed up that price with force. It's a price the SU can't afford to exceed and the colonies can't afford to get because, locally, the value of those goods has skyrocketed as they exhaust them, either by using up rare resources of by being unable to produce more without further investment.
The "abandonment" phase comes when a colony's resources have become worthless on the market because other, newer colonies are better at producing those resources. That means older colonies can't trade the goods they have for the goods they need. The Free Colonies have started setting up a parallel economy, the existence of which is a direct threat to the SU's power structure. It will have (is having? How's this going on Orion 6?) the effect of increasing the prices of the goods the SU needs because there are other buyers — other colonies. That's a direct threat to the SU's ability to continue to feed its people.
The Ijad factor into this because the only thing stopping them from being like the SU is an ethic that hasn't been challenged for millennia. The last time they had an emperor-like entity, it went very badly and they learned the lesson. No one has been able to succeed at such a venture since due to their religious prohibitions that just keep such ideas from ever catching on — until the colonists showed up on their doorstep. Let's see how long their resolve lasts. What they want is to be as big a pressure on the SU as the SU is on them so it simply goes elsewhere. That means establishing colonies in other solar systems...
Transit gates are like little colonies, yeah. Think of them as port towns; reliant on shipping for all of their goods but those produced by their immediate neighbors. When you use a transit gate to send matter or data to another solar system, it's pretty imprecise. You probably don't exactly hit orbit. And you sure don't want to try to get so precise that you lit atmosphere because who knows what your incoming velocity will be. So you go for moon distance and see what you get. Having another transit gate at the other end makes it far, far more precise. A solar system with a transit gate can bring in materials bound for any body in that system. They take years to build, yes. When a colony is established, it's the first thing a colony has to pay for. They have to feed and equip themselves, too. So they start off in debt, but with the promise of riches of their newfound colony. They start producing to get out of debt, then find the value of their output dropping, and still have to support the colony, its expensive transit gate, populated and operated by locals but policed/defended by the Terran Transit Marines. I'm sure some colonies are now considering scrapping their transit gates for the resources they contain, which furthermore makes attack less practical, but eventually those colonies will be lost to history, unable to even communicate between the stars.
Can't write more now. Sorry, guys! I hope this gives some clarity!