> I also don’t believe that fission, fusion or antimatter are on the same branch of the tech tree.

I mean, you need to radiation-harden your structural materials and electronics for both fission and fusion, and some hypothetical antimatter reactor is probably using an evolutionary version of EM confinement techniques from fusion.

But, yes, fission reactors are ultimately kind of ooga-booga (put a pile of metal in some water and run a steam engine) that's hard to characterize as on track for something sci-fi like an antimatter reactor.