You picked one of the most difficult pieces of code that exists. The road map is something like
- Be fluent in Java.
- Be fluent in programming.
- Be fluent in program design.
- Be fluent in concurrency.
- Be fluent in networking.
- Be fluent in data bases.
- (probably) Be fluent in real-time.
- Be fluent in system design (mixing all the above, and understanding the advantages and disadvantages of all choices that exist.)
Here, "be fluent in", means if there is a question about a topic at this forum in the advanced sections, be able to give a proper answer.
The problem with the roadmap however is that you likely don't understand the words that I use. I don't know how to explain them better either.
A second problem is you haven't reached "8" yet, which means you picked a goal without understanding the implications of that choice. In other words, maybe your dream game server is better constructed in a totally different way, using completely different parts. If the system design changes, the required knowledge also changes, and thus the roadmap changes.
As for more practical advice, I'd suggest learn Java, programming, and constructing programs from problem descriptions for a few years first.