Learn by doing projects, start by writing simple programs that cover your personal needs or needs of others, that's probably the most fun way
>This mindset is completely wrong, go with whichever you enjoy and feel most comfortable with
That is correct. Doing something you like means you'll be eager to spend time getting good at it.
>besides, Java is a dying language
That is irrelevant. I don't know if it's dying or not, but enterprise has tons of code that's written in Java and already existing Java teams, and those things are not going away anytime soon.
Also, you need only beginner level knowledge (core concepts, syntax, most widespread libraries) to get a job as junior. Intermediate skills are mostly un-fun and disciplinary (writing tests and testable code, version control, basic software engineering), those are kind of hard to learn just by reading books, but you'll learn those fast at an actual workplace.