I have a theory about the fuel, can you confirm?
I think that normal ships use deuterium-deuterium fusion because deuterium occurs naturally in sufficient amounts and is relatively easy to refine.
However, because of the square-cube law, capital ships use much more energy, requiring deuterium-tritium fusion, which produces more energy per ton. This wouldn't be financially viable for smaller ships since tritium is harder to obtain and decays with a HL of 12.32 years, but larger military powers can afford it.
Also, as mentioned previously, the new fuel can be obtained by mining, which fits this theory since it can be 'bred' from lithium, which can be extracted from lithium hydroxide that you can mine from icy rings.