Probably a bit of an extreme example, but the "X" series would be a great sci-fi option. X4 Foundations and it's associated DLCs is too much content to fully grasp honestly. I played 50 hours of that game and only understood a miniscule amount of the scale and stuff you could do in it. They built it as sandbox as possible from the ground up so pretty much anything you could think of doing in space you can. All the while the universe is evolving around you whether you influence it or not.