I've been publishing third party content supporting the Starfinder Roleplaying Game since the first month after Starfinder release, having created 9 game supplements and modules supporting it so far. As third party, because I'm not allowed to publish using the default Pact Worlds setting, all my modules and supplements include elements of our own 3PP Starfinder setting. I've published Dead in Space created by T. H. Gulliver who created the first ideas in the development of the setting used in all our products. I also released The Planet Builder, a small supplement co-written by an astrophysicst to create scientifically viable, entire star systems to create mini-settings using a series of tables and a planet point system (and stat block). In 2011, my Starships, Station and Salvage Guide rules supplement was the most popular Starfinder 3PP sold on DrivethruRPG for 6 months. I'm currently working mini-setting guides supporting the Kronusverse, and both Corporate and Colonial Space as our own default setting. In the first month following the release of Starfinder, we released a FREE one-shot module, which is a bit dated now, and hasn't been upgraded, but was a great module as a start to a potential homebrew game, called Rude Awakening, that is still very popular!
Right now I'm learning Pathfinder 2.0 rules so I can be ready for a future edition of Starfinder, which was announced for an update to Starfinder in 2 years, will be redesigned to be more closely aligned with Pathfinder 2.0 - I want to be ready when the conversion finally happens.
Though I'm biased, and though I have preferences for harder sci-fi game systems, I've thoroughly enjoyed my development and content creation supporting the Starfinder RPG, and will continue to support with my third party content.
There is a lot out for Starfinder 1e. It is basically Pathfinder 1e fantasy in space with slightly toned down caster classes. If you are familiar/comfortable with 3e/PF1e then it is a good option for a spelljammer with actual tech type of game. A ton of material is srd/OGC and available on Archives of Nethys so you can work off that if you want and get all the rules and races and classes and tech and monsters and mostly everything but the setting and adventures.
The rules PDFs used to be a fantastic deal at $10 each, they bumped everything up a bunch though in the last year or so.