I'm going with boron. It has the downsides of mostly being a mined material, but if I stick with the idea of Oberon station as a wormhole tap (for ridiculous amounts of power) you can still make the stuff the hard way.
In fact, the whole boron chain has some cool world-building aspects to it. First, if you've got the infrastructure to be pulling the raw stuff out of the ground, you can be using it in various high-tech materials; boron-fiber composites, borosilicate glass, etc. For fuel use, it has to be refined. It has to be extremely pure, and you need 11boron and leave the 10boron alone.
Which is easier than U238; with an atomic number of 5, the difference of a neutron or two makes a significant difference in mass. There's a couple different chemical isotope exchange distillation processes, the second one a "greener" one (aka fewer nasty acids to deal with). Or there's laser distillation.
And for that matter, you can breed the stuff by slamming a proton beam into oxygen-16. And I don't have any idea what the surface of Oberon might be like (it is an exotic star) you can always find some water ices or molecular clouds when you want them.
So that gives us three nice layers of tech. Oberon Station could be producing Boron out of pure, err, interstellar gas, purifying it with more lasers, and sending it straight (because it is purified in basically the same sequence of operations) to packaging up as starship fuel.
On other industrialized colony worlds, mine the stuff as borax or whatever (a lovely thing about boron is just how hydrophilic it is, meaning not only are there beds of concentrated deposits, but these are happening on planets with a water cycle (that is, probably habitable ones). And being as these are heavily industrialized colonies, those dark satanic mills can run the stuff through chemical distillation. And, as I said, turn a bunch of it out to local industry.
And then you have the worlds that could happily mine you some borax, but a twenty mule train isn't adaptable to the task of purifying and distilling the fuel, meaning you are getting the thing I wanted; the high tech worlds at the center of the web are exporting the power that makes the trade lanes possible.
And the stuff is a stable isotope, if it wasn't for oxidation and other potential contamination, you could shovel it like coal. Or like cocoa; pick your allotrope. How it is packaged I don't care but the only part of it that calls for special handling is the purity, and the consequent price.
And when you get it into your engine room, outside of confinement questions (they are still playing with inertial confinement) the short version is you superheat it into a plasma then fire an accelerated proton beam at it. Which you might be getting by blasting some hydrogen feed stock. And then the output is mostly aneutronic and as such not only less of a hassle, but you can use direct electrical conversion instead of running a steam cycle.
(Mostly. Biggest problem is 10boron poisoning. You are breeding the stuff in the reactor and you've got to get those products out of there because they do reactions which are much dirtier.)
(Which also might be with lasers.)
Today I also had a wonderful bit of serendipity. I was writing a test draft of the first scene. It felt like I needed to name my protagonist's home world. So, Keri Sinclair comes from an "ag-world"; a farming planet, breadbasket for the more centralized and population-dense worlds. They are your typical poor-but-proud. And as is already noticed, there's a bit of Celtic (and English) mythology going on.
I was thinking about Stirling Bridge, but I wiki-walked in the wrong direction and ended up on Bannock. Which reminds of Bannockburn. But is also a bread eaten in Scotland.
Oh, yeah, and connected to medieval Scottish breads there is some wonderful stuff about the manorial tenant farmers of Scotland, all sorts of thirlage and multure, grassum and quern-stones. And I'm probably not going all crazy with Scotts terminology and culture for Keri's world -- it is enough they are thought of by other worlds as hayseed farmers -- but that vassalage et al gave me a direction for her character other than joining the Navy (that is, the merchant fleet) for the education and a lack of any better options.
No, she joins because her people consider it an honor and an obligation. She doesn't need to find it later, translating from her dutiful role as middle child seeing after the young ones after the oldest children have gone off to college. No, she can walk in intending to serve her duty to the best of her ability.
The only problem is her idea of what that duty means may not quite match up with that of some of her bosses.