While I in some ways prefer Pathfinder, Starfinder is likely a better fit. The main thing I don't like is the assignment of levels to equipment, even though they are mostly guidelines. There are also guidelines for adapting material from Pathfinder, which could be used for other d20 games as well I think. There are already number of conversions for Pathfinder, so there is less need to reinvent the wheel.
The rules for monster creation should make it easy to adapt a number of creatures for Starfinder. They have stats for generic predators, for instance, which you can then adjust to fit whatever concept you need. For instance, you could add a Zerg Graft Template to turn it into a zergling or maybe broodling.
I haven't actually played Starfinder, so I don't know how good it is in play. I mean it is d20, for good or ill you should know how it'll go.