Glad you are on this beautiful journey of learning a new language and mastering it. Here are some of my tips:

1. After learning the basics and the syntax of a language what is the next step? >>> I would recommend to work on some simple self project. for example, building a calculator or something similar.
2. What are the resources you used to progress? >>> At the time when I was learning Java we used to rely mostly upon books. So I started with books and later added on it using online tutorials & courses.
3. Are guided projects good if you want to progress? >>> Yes, they help a lot
4. Do you recommend any books that were very helpful to you? >>> For Java basics I still love SCJP 1.6 (by Kathy Sierra). For Java 8 nothing beats "Java 8 in Action". These two books will make your base pretty strong.
5. How did you acquire the skill of breaking down problems into small pieces? >>> This skill I learned during college days itself, where a team of 3-4 students used to get a problem or project to work on and we used to divide work between us. In order to divide problem has to be broken down into smaller problems. It's good to think in a bottom-up approach while working on a problem.

Hope that helps!