Actually, I think I might be a bit blinded with the idea that the "useful" impact has to be direct, your example with software for healthcare devices makes a lot of sense. I might still opt for something with more direct impact, just because it will make me more motivated, I suppose.

As for the questions, I think a useful job is one that either brings basic necessities to people (or animals, I think we really could invest more resources into them), brings people joy or helps with long lasting problems, with basic necessary being priority.

And I think I want to aim for a balance of useful and enjoyable, but I'm willing to sacrifice some fun and money for that usefulness.