With that set up, it's sort of a gimme that there's going to be some kind of stargate system. And the older than the universe indicates that something has roundtripped twice which suggests time travel. Thus, we are not particularly surprised (hey you! You do realize I include spoilers in my reviews, right?) that the climax of the book revolves around whether a time paradox exists and how it will be resolved.
The choice Meluch made for solving that little conundrum can only be done once in a series. At most. I, personally, kept thinking, dude, that was just a dressed-up it-was-all-a-dream, wasn't it? But the new-real universe at the end may include some interesting story possibilities. Now I have to decide whether to continue with them or not.
The whole recreate-the-Roman-Empire thing has been done before, altho again, Meluch's take has some entertaining aspects to it. Marines being carted around the stars by a Space Navy of the future has also been done (to death, arguably. Heh. Uh, sorry about that.). Serviceable (okay, I can stop any time) milsf for those with a weakness for that kind of thing.
Separately, I really need to remember to call K. and have a long chat about the romantic entanglements of this novel. Because those, honestly, may have been the best aspect of the whole thing. ("I'm not cheap. I'm free." _That_ is a fantastic line.)