For the longest time, I was primarily focused on the Spring ecosystem only; that's why most of our courses are all about Spring. 
And, yes, that made Baeldung one of the best ways to both get into Spring Boot but also explore the more advanced aspects of the framework. So, overall, I think it was the right decision, but...it wasn't enough. 

Spring is a core part of the Java ecosystem and that's only getting more and more true. But, there's so much more to Java than just Spring. 
A strong understanding of testing, with JUnit and Mockito. 
A practical use of Maven to build the project. CI good practices and Jenkins later on. 
Deeply understanding collections and streams in Java. IO. Concurrency. Yes. 
Good logging practices, which is one of the most underrated skills a developer can learn. 
Basically, the reference platform for a Java developer: 
>> The Course Roadmap

And a simple way to access everything, all under All Access. 


Hope that  helps. Let me know if you have any feedback, of course. 
Cheers, 
Eugen.