Starsector is described as '(formerly “Starfarer”) is an in-development open-world single-player space-combat, roleplaying, exploration, and economic game. You take the role of a space captain seeking fortune and glory however you choose' and is a role-playing game in the games category. There are eight games similar to Starsector for a variety of platforms, including Windows, Steam, Linux, GOG.com and Lutris apps. The best Starsector alternative is Qanga. It's not free, so if you're looking for a free alternative, you could try AstroImpact. Other Starsector like games are Star Citizen, Astrosphere, X - Space Game and Space Rangers.
I like Astrosphere as an alternative to Starsector because it offers a similar mix of space exploration, ship customization, and tactical combat — but with a fresh focus on building your own space station and managing resources.