I must confess I left Stellaris behind too after the Megacorp release. It was a culmination of several issues. The game is still not feature complete 3 years after release (still no intelligence layer.) The diplomacy has plenty of features but the AI barely participates, other than to just set diplomatic status according to its like/dislike score. The gameplay systems seem overly restrictive (forced end war, can only declare on certain people, and so on) which leads to a gamey feel that I didn't expect from a sandpit style game with huge galaxies. There are too many abstract resources to balance that add to the gamey feeling. The DLC have added additional playstyles but in effect they have added multiple ways to do things with a lot of overlap, that seems to have bloated the feature list without really enriching the strategies you can get involved with.
The DLC policy has contributed to a feeling that Stellaris has been an ever-changing beast... the game is designed a certain way, it settles down, some nice mods appear on top and it feels pretty stable. Then a major update comes out, the mods don't work, some central game functions are completely reworked, many critical functions and gui items cease working, and everything feels like it's regressed for a few weeks while official patches and mods scramble to become stable again. It seemed ridiculous to me that for significant periods of time after release, major parts of the game were not working at all. Or like with the megacorp release, the AI was completely non-functional for weeks after release. You've gotta have a lot of faith in the game to overlook things like that and trust that it will eventually get there - for me though I just felt that this was unacceptable for a released game. There are definitely some positives to the Paradox/Stellaris business model... some of the reworked mechanics are clear improvements to original versions. But I have ended up disillusioned where things feel too much like they are engineered to keep customers in the pocket, paying for updates for the life of the game.
Much as that was a bit of a rant, there's a lot to like in the game. It clearly has the best space 4x flavour around imo, with nods to pretty much any sci-fi book/TV/film that you happen to love. If somehow you are into multiplayer then the Stellaris engine is great for that. I know the game makes a lot of people happy - I think you have to be the type of player that will happily accept the mechanics and the dev cycle the way they are, and be in it to enjoy the ride.
Edit: oh - check out the Starnet AI mod. It's a rework for the AI players that makes them a lot more like a multiplayer game (emphasis on military techs, maintain large fleets and swallow up weaker neighbours with no remorse.) Several people have said this AI is genuinely a big threat. No idea as to the status of the mod with regards to the latest DLC that came out, but the prospect of AI players that can actually kick your ass is pretty inviting.