I'm not sure if you're working from the right definition. At least by revenue and number of employees, SpaceX would still be Mittelstand, I believe. If you're thinking smaller, the term "lifestyle business" was once coined for such endeavors by, I believe, 37signals.
As for the larger scale: every startup is expected to settle into some size with little further growth at some point. The "unicorn" is a rather new trend, and it's only a small fraction of even VC-funded startups (although it's a larger fraction by funding or attention).
According to German Wikipedia, SpaceX absolutely does not correspond to the definition (suggested, because there is no official one) of what "Mittelstand" is. That means less than 500 employees and less than 50 Mio. € revenue, family business or at least involvement of one founder, and most importantly (called out as such) economical independence of the owners.
Or is this a case where the German word has been co-opted for a different meaning in English? (Though other words like Kindergarten or Zeitgeist are very close to their original meaning, now that I look them up.)
> I find the English usage of “Angst” really confusing, as it really just means anxiety or anxious.
USA: While angst and anxiety come from similar roots, angst is not just anxiety but a dread or anguish. Often angst is focused in a certain way, such as towards a particular condition one person has or a group of people have. Anxiety is more general.
There may also be differences between the two words in certain fields, such as with professional definitions.
There are also other derivative uses, such as teenage angst, that are more akin to feeling overwhelmed but not necessarily anxious.
As for the larger scale: every startup is expected to settle into some size with little further growth at some point. The "unicorn" is a rather new trend, and it's only a small fraction of even VC-funded startups (although it's a larger fraction by funding or attention).