After his people were destroyed, a space-faring cybernetic gun-wielding samurai hunts down a powerful old enemy to avenge his people and prevent a devastating alien weapon from destroying all life.

>>> space-faring cybernetic gun-wielding samurai
This is a lot of unnecessary adjectives where, ideally, a characteristic that tells us about the spiritual journey the character goes on would be better placed. It’s always tough with science fiction as there’s a lot of world-building that needs to take place but I think in your logline this isn’t as relevant as the character. In my opinion, a samurai needs a samurai sword… samurais are historical warriors and while updating them is great fun, they still need something that keeps them classical and identifiable as samurai.

>>> powerful old enemy
Is the enemy old in years? Or a long-time enemy of the samurai? It probably needs clarification that he’s the one who destroyed the samurai’s people.

You currently have two goals. Avenge his people and prevent the weapon destroying all life. Which is the primary goal? Also, when you say ‘all life’ do you everyone and everything in the entire universe? I can’t help but wonder why an enemy would want to destroy all life. What’s the motivation behind that. Most bad guys have a motivation for killing people – Thanos is a perfect example (no spoilers though). I’d consider having him just choose to avenge his people OR prevent the bad guy from destroying another planet. That’s not to say the other can’t happen – by stopping the bad guy you’re preventing future harm and avenging his people BUT for the sake of the logline and how the readers sees the protagonist one tangible goal is important. If he’s avenging his people he could be seen as more angry, possibly selfish, whereas if he’s preventing further deaths he’s more noble and selfless.

Hope this helps.