So how do you decide what system to use when you know so many? I still default to dnd but I wanna branch out, specially for more sci-fi stuff
I only know a fraction of the games I own xD Some are easier than others though. I take recommendations from friends, youtubers, and the people who work at my FLGS. A lot of time though, I'll just see a book and say, "That looks cool" and buy it. But I haven't read all of them yet. I'm working on it though!
Last year my DM wanted to take a break from his campaign and be a player. I offered to be the GM for the group but I wanted to try out Starfinder. I had run it once before and offered to run it again as a one shot to see if the group would like it. They did so I started up a full campaign. Starfinder was an easier switch because both games are D20 based, have the same stats, and action economy. As we've played over the past year we've made a ton of modifications to the rules. It's been working out great so far!
Now that worked because I wanted to run Starfinder, my group was interested in trying it, and we've made the system work for us. That might not work for you for any number of reasons. I know you want to run Sci-fi but what style of game do you want to run? Are you more into tactical combat, roleplay, world building, or narrative? Realistically any game you choose to play can fulfill any of those goals but some are better than others.
What style or styles of game do you want to run? Are you going to do more horror, sci-fantasy, space exploration? Want a pre-established setting? If you don't want to heavily invest in a game system that you don't know will catch on I suggest looking for starter sets or quick start guides. They're usually cheaper in physical and sometimes even free if you get a digital one.