You cannot conquer other systems, so war is out of the question. Settling more worlds and expanding my colonies feels like a waste of time, as it will turn around and bite me on the back with colony threads and lowered colony profits.
Well, in general, playing a videogame is a "waste of time" if you look at it that way, but I do it because it's fun. Once I've got a colony with every industry for total self-sustainability in my faction, yes, settling more planets will technically reduce my net income per planet since the markets don't expand. How much does that really matter, though? If I want to colonize every good planet in the Sector, what do I really care if I will make a few hundred thousand less credits per month by doing so?
Also, you can't technically "conquer" other systems, but you can certainly decivilize them and take their planets for your own. For me, the endgame in vanilla StarSector is just, whenever I've finished the "story" I've decided on for this particular playthrough, once I've completed all the arbitrary goals I set for myself, then I start over with a different gimmick. Next time I might be fervently loyal to the Persean cause, or I might try to get in good with the pirates and corner the market on space drugs.
Now there's mods that add more endgame activities as well. I'm not a Nex guy but as I understand it it literally turns StarSector into a 4X style game with the systems changing hands in wars between factions and whatnot. Myself, I have a couple of favorite mod factions with optional endgame challenges. In a modded playthrough, it's not done until I've tested my might against the horrors that lurk in Seele (from the HMI mod) and Uvas (from the Xhan mod), those guys make the Hypershunt fight look like Spongebob vs The Muppets.