I agree with your first point. But I think the criticism you direct at Rebel Moon is maybe too general — and could just as easily be applied to a whole host of properties (some of which you probably like).
Fact is, the “science fantasy/space opera” genre is often a catch-all action/adventure category that qualifies as “sci-fi” because the settings happen to involve space travel and alien planets. And these tend to be amalgams and pastiches of other genres and traditions. E.g., the original Star Wars was an homage to 1940s sci-fi serials. Star Trek was “Wagon Train to the stars” (or, alternatively, Horatio Hornblower in space). Alien was a monster/horror story set on a spaceship; Firefly/Serenity was, quite explicitly, a Western set in space. Etc. IOW, much of this genre involves a lot of borrowing, recapitulation and repurposing of prior material and archetypes.
So, yes: Rebel Moon — a sci-fi actioner inspired by Seven Samurai — hardly sounds original. But there aren’t many examples of sci-fantasy that are truly original. Most are just variations of the same thing.