Those are some really good words! In my school experience, the equivalent of "överkurs" was "bonus" or "extra" - "Read chapters 2-6, 7 and 8 are extra" - but överkurs is nice since it is a bit more specific. I do think that moral texts should stop saying "supererogatory" and just say that "doing more than is morally demanded of you is bonus", but philosophers would have a heart attack.

And unlike in Swedish or German [making compound words is] not something everyone feels free to just do at any time.

So English actually does have this, it just is treated differently than in German or Swedish. If I refer to "printer ink", a "toaster oven", "bonus points", a "coffee table", an "airport terminal tram", or a "conference room television", from a linguistic standpoint those are the same thing as when German or Swedish has a compound word. The randomness of developing a written form of our languages just wound up saying that English should put spaces between the different components.

The article says "Yes, you can make two- or three word phrases but those are the second-class citizens of words", but many (perhaps most?) native English speaker would consider those to be first class citizens, especially the common ones. Or if they would not say that, the language portions of their brains probably still do.