Imho, the java aspect of it matters much less than the backend concepts. Are you already familiar with those ?

There's quite a lot of stuff to learn on the backend and it really depends on which layer you want to focus on. If you're interested in developing business services then i would recommend writing a restful API (for example for a to-do app) with spring boot and your preferred flavor of SQL database. That covers a lot already. From there you can look at how to scale performance up (caching, queuing, asynchronous).