Oxygen Not Included and Surviving Mars are both great colony simulators.
Avorion is a good space empire game (fly a ship, build a fleet to follow you, build stations, etc) with custom spaceships you or those on Steam Workshop can build.
Hearts of Iron IV and Stellaris are both great grand strategy games. With Civilization VI being the genre-defining 4X game.
Subnautica is a great underwater exploration game. Amazingly done visuals and a solid story to bring it together.
DCS is an amazing combat flight simulator. And it is a simulator, not an arcade game, don’t expect it to hand-hold (though there are a few modules that are intentionally dumbed down to help out new players). The US Air Force literally uses it for training new A-10 pilots.