Unlike other versions, the game has no achievements and has a lot of DLCs. Having a lot of DLC isn't necessarily a problem, but if the game had achievements, it would even encourage me to buy the DLCs in the future.

I should have researched it before activating it, but anyway... That was all!

Now that I saw that the game is on PC Game Pass. I have signature.

you need a Paradox account to unlock achievements with an ironman game

the fact that you got it on sale from someone else has nothing to do with anything but have fun

That was just a rant. I like achievements on GOG, although I'm a minority here. And I have Paradox account.

I'm thinking of playing on Windows 10 while the game is on Game Pass. I didn't understand that part you wrote.

It matters not where you got the game. The only achievements available are through using the paradox launcher and logging in.

I am not aware of any achievements through using Galaxy as I do not use Galaxy.

So if I play on Steam or Windows 10 or GOG itself, are they the same achievements? Like what happens to Xbox games on Steam that need Xbox Live? Example: assuming I had a copy of Steam and Windows 10, the achievement that I got one would automatically get the other?

Taking advantage... If I buy DLC from one store, can I use it in another because the game is basically the same?

I just tested this: I have Stellaris with a bunch of DLC on Steam, and I just got the base game on GOG. If you copy the DLC files from the Steam installation to the GOG installation, the game will recognize them. So DLC can be used across stores if you manually copy the files.

if you install Stellaris Steam on the same system as Gog i.e, both at the same time then they fight so remove one after playing around with files

if you buy the base game from one platform then you need to buy the Dlc from the same platform, they don't swap from one to the other
you buy from Gog = get a Paradox key
you buy from Steam = get a Paradox key
in the past buying from Paradox got you a Steam unlock key... as far as I know that's still the case
be aware that many of the other platforms around are selling a console copy of Stellaris which is a limited scaled down build and not playable against people with the pc build

achievements on Gog, Steam or Paradox are the same and Paradox controls them so Galaxy or Windows 10 has nothing to do with it... the achievements in the console builds are mostly the same but there are less of them