Personal opinion is all of them.

Yes, there's some skills which you will only encounter, if you're actively building up a larger project. But you can't just stumble into a larger project and learn all of those, you do need to actually (help to) build it from the start. And that takes a lot of time.

Having a larger project going on, on which you're motivated to continue building every so often, that's really good. But you'll need to do some smaller projects every so often, too, to keep up motivation and give yourself a chance to start fresh and maybe do things differently.

In particular, small projects also allow you to spend a completely unreasonable amount of time on some specific topic, because the scale of it won't be as big of a multiplier. And adding those topics together, you will have a lot of the knowledge for a larger project.

Particular topics, which you should spend a completely unreasonable amount of time on:

• unit tests, integration tests
• error handling, logging, monitoring
• a build system, CI/CD
• refactoring, structuring a larger code base, multi-project builds
• TLS, authentication/authorization

(Do not try to do these all at once. Pick out one or two new ones per project.)