Personally, I've not found anything that is super close to this game... But Jupiter the Nexus Incident has some similarities, it is a tactical space game, mission based though. You can find it for pretty cheap on Steam.
If you don't care if it is space based or not, the game plays a lot like Mound and Blade: Warband. Which is a medieval game similar to Starfarer's campaign. I recommend playing with the Prophesy of Pendor mod if you try it out. Another cheap game to get on Steam.
SPAZ is similar, in a sense, and worth playing...
Have you ever tried Jagged Alliance 2 with the 1.13 mod/patch? It is again not space based, but is a tactical turn based game with some similarities to Starfarer...
Gratuitous space battles has some similarities as well, but in that one you setup a "mission" and then hit play and see how it works out, so a bit different.
Otherwise I gotta say that Starfarer is pretty unique...