scratches the space economy itch

Ah, gotcha, fair-enough.

But yeah the fleet combat part of X4 never quite got me interested beyond “if I make 100 fighters, I should be able to take out pretty much anything”.

Ah, okay. I was just trying to figure out how it differed from some of the other things you listed, and fleet combat was one. But the economic side is another, and, yeah, I can see the economic side of the X series being appealing if building a big space empire is a goal.

If you're looking for a space economic sim, that's entirely-absent from NEBULOUS, and in fact they even mention that up-front on the product page -- they're going for combat simulation. So it won't fill that slot.