I find Subnautica has less replayability than other survival games since the map and questline is static. Once you know where everything is and you’ve seen all the plot beats there’s not much reason to play the game again unless you want to challenge yourself with a speedrun or, as you said, one of the harder difficulties.

I wouldn’t consider creative mode or sandbox mode to be a core part of the game. They’re great for fucking around or as an extended tutorial, but I see them more as external tools than as part of the game experience proper.