My first teachers were anxious that I should write Latin that was plain and simple; later they allowed more relaxation of style to a degree where they felt I could handle the idioms comfortably. I constantly remind myself of this. There's seldom just a single way to express yourself, and it all finally depends on your style. In Latin, just as in English (and, I imagine, in most languages), you can be quite economical with your words (Tacitus generally) or orotund (Cicero). To answer your particular query:
If you are simply claiming to do any kind of job, 'work' is implied in the title. This makes it much simpler. In your first example, pedagogus sum would be quite adequate: it just as much means 'I'm a teacher' as 'I work as a teacher', or 'my work is teaching', etc. Similarly, if your nephew wants to be an astronaut, there's no need to imply that he will work when he becomes one. But if you need to say something more explicit, there are alternatives to such bald statements. You might say habeo statum pedagogi, for instance. If you are a barrister representing an orphan in court, you act in loco parentis. If you want to emphasise the duty, then construction with fungor is fine (I wouldn't be surprised to find Cicero using dum consulatu fungor rather than dum consul sum). There is also that very useful little adverb qua, which has a range of nuanced meanings, including 'considered as', etc.
And if you want to extend the subject, then an ablative absolute is often appropriate. me pedagogo discipuli bene discunt, 'When I'm the teacher, pupils learn well'.
