I run a lot of Savage Worlds. I adore Savage Pathfinder, if there were a Savage Starfinder, Pinnacle and Paizo could just take my wallet entirely.
Savage Worlds is a good game for a younger crowd or mixed crowd. It’s not super deadly. It’s swingy, and the highs can be very high indeed. It's easy to teach, is still tactical, non-combat characters can still be productive in combat situations, and has decent tools, chases, dramatic tasks, quick encounters, and so on.
I don’t find Savage Rifts much more complicated than Savage Pathfinder. Both are on the crunchy end of Savage Worlds.
I think I’d recommend something like Flash Gordon for pulpy sci fi. It was released for the previous SW edition but there’s a conversion document and it’s 90% there for SWADE right out of the box, since it was so late in the prior editions life cycle.
It’s also very easy to take your favorite Sci Fi setting and drop it into Savage Worlds with minimal fuss. I’m working on Coriolis now. The fan Savage Star Wars book might be the best implementation of a Star Wars game ever. The prior edition Science Fiction Companion does most of the heavy lifting.
You can typically find a decent Savage Worlds clone for whatever you want - Gamma World? Darwin's World for Savage Worlds comes close. Teenage Mutant Ninja Turtles? Use Big Apple Sewer Samurai for Savage Worlds plus the Super Powers Companion, and so on.
On and off I’ve been toying with Savage Starfinder using Savage Rifts as a foundation and porting over something like the spaceship rules from Starfinder itself and just translating them into Savage Worlds terms. you could pretty easily handle the starship piece with the chase rules though I think.
You mentioned having a bunch of Free Leagues stuff (same here) maybe try Mutant Year Zero and lean into Fallout or Gamma Worlds style tropism?
