Endless space but a little bit deeper though. its only complex if you don't want to learn how a game works. I won Ck2 4 yrs ago and fired it up a few times but always quit instantly due to the learning process. 2 maybe 3 years later i was at a friends house and he doesn't have a great pc so i was limited on what to play and decided to give ck2 another go. First playthrough was a disaster, doing everything wrong. But i started learning the mechanics and right now i can without doubt say its the best game i have in my library. Same with EU4 although that went easier, thanks to the knowledge i had procured from playing ck2. My point is: If you're looking for a fast game, in which you can jump in and know everything in a few hours, stellaris won't be the game for you(or any other grand strategy). But if you're willing to learn how the game works, what "secrets" there are available, then you will have a blast with it. This is exactly why we need demos though. Games like this should be tested before you buy it, as its really a niche.

Stellaris is nothing like sins of a solar empire (which is an RTS).
Endless space is very similar to CIV but with more focus on war.
Stellaris is closer to CIV then endless space (because for stellaris war is not something cheap which means you won't have a fast pacing game once you get "landlocked")
You don't have to worry too much about the complexity because all factions start with 1 planet then slowly expand.

Think about it like this: once you discover the 2 or 3 factions around you you will probably learn their behavior before you enconter anyone else. Just so you know the behavior is shown in the diplomacy screen when you are talking with a faction, each faction can have at most 2 of 8 behaviors (called ethos in game). if their ethos matches yours they will want to be friends, if their are opposites they will have a rivalry with you. those are straightforward but the cool stuff happens is with those that are neutral, which may be persuaded to join you, persuaded to fight against you or they can be neutral and neither your or your rivals can persuade them to join.
What stellaris does real well is introducing the mechanics slowly because of that 1 planet start. At first all of your species will be united under your banner, as you colonize planets away from your capital the ethos of your population might change which means people might ask for better living conditions (if you are treating them badly), to change of policies (for example ask you to not bombard enemy planets to kingdom come, for alien spices to have the same rights as your main species) .

All that said you don't really need to "understand" the game, because most features can be automated by the AI using sectors (currently these sectors are not really as good as a player but in a large empire they work reasonably well

Most of the other strategy games have the "problem" that in mid-late game you are on a set course for a determined type of victory and the game is just boring work to tick the necessary check boxes to win, in stellaris it is there that the complexity starts to kick in so that you don't steamroll everything. A few examples: a revolt in your empire because of all the slaves you have, a few spiritualistic factions have united as a federation against your aggressive materialism, they seek to enlighten you (and remove the droid population that is boosting your mineral output)!, a signal from out of the galaxy is detected, what sort of faction can travel between galaxies? but more importantly is it friend or foe?

In a nutshell Stellaris is a slower game with more roleplay ability but its complexity isn't too far from CIV/endless space/endless legend, existing mostly in mid/late game.

Oh, thanks eev for your concern.

Btw, do you fancy Doom? :)