I was about to buy, but I read a couple of reviews on the game pages that the DLC isn't available on Galaxy. Problem is: I'm on Linux machine and so don't have access to Galaxy.

Has this changed, or is it going to at some point?

Yes, the DLC is available without Galaxy. Blue post (in German).

I assume that the offline versions of future updates will be delayed compared to the Galaxy versions, but so far everything has been released in an offline form.

Thanks for the fast response.