SF.

Yes it may have a fantasy feel and edge but it’s still SF. There’s no need to break down everything into multiple genres all the time, and despite Lucas saying it is also a Space Western it isn’t a Western either despite having some identifiable tropes.

There is realistic background in the OT for most everything and if you’re curious you can research the methodology of how things work in universe. I still think the used universe idea in the original helped not only ground it for a audience but also made it possible to bask in an SF landscape.
Reading the novelization feels like pure SF with some fantasy elements.