It hasnt been established that the Titan Thanos is from is the same moon of Saturn named Titan in our solar system in the MCU. As of now that is a comics exclusive location.
MCU Titan is definitely not Saturn's moon. How is this a serious question?
Thanos described Titan as a planet. Not a moon. It even HAS its own moon, which Thanos threw chunks of as an attack. Saturn's moon Titan does not have it's own moon.
Thanos also described Titan as "beautiful" and showed what it looked like before disaster. Saturn's moon Titan is an icy ball with methane seas with a -200 degree F climate.
He also describes it as resource strapped. Too many mouths with not enough to go around. That race was clearly Spacefaring. If Titan was THAT short on resources, raiding Earth for them or colonizing it (Because Earth is MUCH larger than Saturn's moon) makes more sense than starting a resource war that kills all life.
Thanos committed to wiping out half of all life in the Universe after that, which we see a glimpse of when he kills Gamora's people.
If Earth was the closest habitable planet to Titan, Thanos would have started there, instead of running halfway across the galaxy.
There's no scenario in which Titan as Saturn's moon makes any sense.