No it's not. You can buy the base game and just stop there, there's nothing remotely compulsory about the DRM. All of it is optional and the base game is fine as-is, especially for $10.

I hope you do realize that Stellaris improved immensly since it's inception 6 years ago?
It's nothing alike compared to the 1.0 version anymore.
So many systems have been deepened and overhauled and with it also the longevity of the game.

The downside is that this DOES cost money.
Development time is not free after all, hence the optional packages.
I agree that this can be seen as a negative.
I wholeheartedly understand though that Paradox charges money for those expansion and packs, since they want to stay in business. And supporting a game 6 years after launch? That's far from standard, ESPECIALLY in the strategy gaming sector. Couple that with the rough start and it's basically a wonder that the game wasn't dropped.

Those expansion and packs have given Stellaris a great deal of diversity of options, leaving aside Mods... which not everyone wants to deal with.

And in any case the bigger DLC's (Apocalypse, Federations, MegaCorp, Nemesis, Overlord, Utopia) are more like expansions of way back then. The smaller ones act like themed packages or add specific stuff.
None is mandatory to the enjoy the basegame.

Friend of mine has just the basegame and is happy as a clam as the saying goes.
Another has some expansions, but wanted Overlord the moment it dropped since he enjoys playing with Vassals.
I on the other hand collect everything... I like to tinker around. I also usually host the MP rounds, which means that anyone who's missing a expansion or pack gets to see the stuff in those games basically.

So it's basically a pick whatever you want option, after all you aren't forced to keep up.
It does help that with each new expansion base features are given for free.

And who knows when Stellaris will be seen as "complete".
Europa Universalis 4 came around 2013... now it's 2022 and it still sees active development.

No. I love 4X and especially Space 4X, but I don't hold with the scummy DLC-scheme Paradox got going. It's nickle&diming people to pay out more than they otherwise would have for fear of FOMO and completeness sake. Sure, consumers should get a grip too, but while you fine the druggie, you imprison the dealer.

This is also the expansion in where they removed white people portraits.
Typical Current Trend chasing greedy corpo doing anything for profits.
Get woke, go broke.

I like to play my games with all DLCs, so I won't buy incomplete games. It's an OCD of mine.

Which of the story expansions are worth it? How is Overlord compared and also the AI ?

The DLCs for Stellaris are incredibly tame by comparison to PDS' previous releases so it doesn't really bother me. They've changed their strategy and while I do think they need to rethink their approach to DLC for the older games, I'm at least glad that all of the DLCs for Stellaris onward seem to actually have substance to them.

"Utopia" gives full access to Ascension Perks and Megastructures. I consider this almost essential. Then again I AM in an unhealthy relationship with Dyson Spheres and Ringworlds. So take that however you want. My number 1 pick.

"Apocalypse" is all about War and shattering planets basically. Nice for warmongers. Introduces Marauders as mid-game threat. Not a "necessary" buy, good flavour though. Probably not to important to pick up immediately. I'd pick it up as a third or so... since the Marauders are really, really nice for shaking up things. My number 4 pick.

"MegaCorp" is good if you want to play as a... you guessed MegaCorp. Ever wanted to play Weyland-Yutani or as a Ferengi? Then this is for you. Unless you want to start with anything trade related this is a safe skip. Pick it up once you want to sell Warp Cores to every Joe and Jane in the galaxy. My number 5 pick... I'm not big on being a Corpo.

"Federations" allows the founding of the very same. There's the Galactic Community which can be played with or taken over... or entirely ignored at your peril. This is VERY nice for a peaceful playthrough. My number 3 pick.

"Nemesis" is all about spying - which I find so-so - and the Galactic Custodianship in face of a galactic crisis... think Palpatine with his engineered crisis. You also can become the crisis. Can be skipped until one wants more endgame content. Comes in last for me... but I find spying not enjoyable. The foundation could also be more expanded, I still find it to barebones. Go loud or flame out. And with this they did latter I find. Mileage may vary.

"Overlord" is all about vassals and some new origins around those, mostly.
Situations are a fine way to force players to not ignore things like energy or food deficits, cause now there's strings attached. And those hurt. Planets might also rebel far more seriously. That was never a real risk beforehand.
Vassal contracts look nice but they need a bit of number tweaking, I guess.
In itself the AI is far more competent, but that's true since around 3.2 or 3.3 in general.
I'll note that some numbers in Overlord probably need tweaking... but that's to be expected.
It would be my number 2 pick.

For species packs:
Lithoid, Necroid, Plantoid, and Aquatic Pack look just suitably different that I find them in general very, very nice.
Humanoid looks what I find boxy and generic enough to pass for human.
In general I like those packs.
You also get some minor perks here and there with them that change how you can play the game.
Not necessary if you start... but rather nice for diversity if you've played for a while.
Love it or hate, up to you. Not needed, but if you want it it's there.

For the story packs:
There's "Ancient Relics"... grab it, thank me later.
That's the one that's just awesome. Gameplay depth, a lot of longevity and over any game you go hunting for those dig sites. That's just plain fun. So yeah, I'll mark it as essential. It's that good.

"Synthetic Dawn" brings another mid- to lategame crisis into the game, IIRC.
It also unlocks robots as playable.
There's a lot of story content in there, robot/synth rebellions and some more.
Not needed, but if you want to play robots you'll want it eventually.

"Distant Stars" and "Leviathan" bring about space monsters, the War in Heaven and more.
They are good. Trouble is I'm not good at remembering at delineating which brings what to the table exactly.
I merged them mostly into one package in my head.
These packages are not needed at the start, they can easily later added. They are really good then, cause they are throwing some mean curveballs.

the word is bugs not substance

Thanks for this. I got the base game and while it was fun, it wasn't entirely engrossing for me. But everytime I looked at DLC threads/options, my eyes would just glaze over and I'd not get anywhere.
Your post is concise (relatively) and informative.