Thank you for your answer @ju52. That solves my problem!
Since I know what kind of part I need to render at a time, I can call Hugo with the correct config-file. Makes totally sense.
By the way - as far as I looked up so far, there is no plan to implement incremental builds, right? I’ve read about some people who use hugo server for that, but I’m not sure if this is “production ready”, and probably not a good idea anyway, since the whole point is to have a static site.