Stellaris! It's a game like civilization, but in space. I have hundreds of hours in and it has been responsible for many nights of me staying up late than i should. Constant free updates and many options for paid dlc to expand your options.
Gaming
If you're willing to wait, I don't think we should leave the upcoming Bethesda IP, Starfield out. It's due in September and from what little I've seen is a real competitor for Star Citizen.....
Except it will be complete.
What, no, it's more of a RPG, Star Citizen's scope (and flight model) is far more involved.
And it being Bethesda, no, it will not be complete. Expect lotsa bugs.
I'm hopeful for Starship simulator. It is not out yet, but it looks promising. I've always wanted a game with a ship with a working crew you interact with or become a part of. I have not ever found anything good enough.
I enjoy X4, but I actually though the exploration aspect in Rebirth was better (unpopular opinion I'm sure). I do like how you can give your captains orders and then just hitch a ride and see how it goes.
Everyone always says FTL, which is fun, but the crew don't really have roles or personalities. They are there and they man the ship, but that's it. And you're always in a rush to do anything which always just ends with a space battle.
Similar for Space Haven. If the crew has assigned departments and ranks rather than just assigned tasks I think it would be closer.
These aren't necessarily top-tier games, but if you want some more unique recommendations: Duskers, Objects in Space, and Rebel Galaxy
Eve Online (I have not played Eve)
Definitely get the Mass Effect Trilogy. You could probably play through the whole thing before starfield comes out if it hooks you like it did for me.
If you like the idea of building your ship and then using it to blast pirates into little pieces, you could give Avorion a go. It’s currently half price on Steam and is a bit Minecraft in space. It had no player view though, everything is done from space craft from mining to trading to combat.
Combat mand flying feels like The Expanse if you have seen / read that.
While not a flight sim I'll give a recommendation for Hardspace: Shipbreaker. Spend some time salvaging old spaceships in zero-g.
Not exactly in the same vain as 3d game but Endless Sky and all the Escape velocity games. Escape Velocity is the grand daddy of all space games, and was even pseudo 3d when that was a rarity for games.
If you want something smaller scale but fun:
Transcendence. https://store.steampowered.com/app/364510/Transcendence/
Also, "Strange Adventures In Infinite Space"
and the sequels to that. The sequel (Weird Worlds) is on steam.
Very short and silly, but fun.