For a longer single player game, you should try Stellaris. If you like the Masters of Orion series, you will LOVE this. You should get the latest expansion, its actually pretty cheap. What is unique about this game? Well, each type of race is played differently and each has its own storyline with a unique endgame crisis and ending. The game also throws plenty of surprises at you at hard difficulty - one minute you might think you´re winning and another minute a whole extra dimensional armada pours down on your doorstep with thousands more ship than you, and even if all the races of the quadrant unite it will be troublesome to fight them(I´m not kidding). It´s slow paced enough that feels like Civilization but its very politically oriented as well. Instead of just conquering empires, you can make them fear you enough so that they split in multiple factions, and one splinter group may become loyalist to you. And a big etcetera... try it if you can, it´s the best long space strategy game I´ve played so far.
On the other hand, if you want a strategy game that you can finish each new game in an hour or two, try Sins of a Solar Empire.
And never forget Sword of the Stars 2... that has the most detailed combat simulation in any strategy space game. Feels like civ, but you can actually play the battles and control the ships.