The word 'sim' is used often for things it shouldn't be used. Orbiter is a space flight sim.
The nearest I think to a 'sim' regards sci-fi space ships, is Rogue System, I would think. KSP with its simple style approach (although not simple in game-play) does a very good job at making the player think about how a space ship should be put together, ready for space flight. Space Engineers does a good job at that too (although a lot simpler in some ways, but better than KSP in others), they're both very good sandbox games.
ED is good at space itself, but so is Space Engine (which is free). ED, as an 'open world' game, is good. Is ED a sandbox, I'm not quite sure yet, but its certainly getting there the more they put in, I hoped it would be a sandbox when I bought into it, so I still keep my fingers crossed it will reach that (just in me view), because for me at the moment, it still falls short of being a 'sandbox' game.
All of the above games, I like. They all make for good games and/or experiences. But don't be fooled where the word 'sim' is used, there are very few actual sim games around.
But the only real space flight sim, is 'Orbiter', from those I've mentioned above.
'Sim', Sandbox' & 'Open World'. Words often used incorrectly in gaming nowadays, just in my view.
Edit: Oh, and NMS for me, a sandbox'ish survival game, with arcade style controls. But very good, nevertheless.