I'm going to turn this question back on you: why do you want to expand your knowledge into other areas?
What interests you?
"Full stack" is an endlessly huge area of learning: distributed systems, databases, networking, operating systems, security, hardware, graphics... it goes on forever.
You have a couple of options that will help drive your learning and keep you motivated through feeling like a complete beginner all over again:
find a problem that interests you and follow it. The right language and toolchain will be either obvious or not important.
find a language that interests you and learn it. The kinds of problems to solve will either be obvious or not important.
Good luck!
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I'm going to turn this question back on you: why do you want to expand your knowledge into other areas?
What interests you?
"Full stack" is an endlessly huge area of learning: distributed systems, databases, networking, operating systems, security, hardware, graphics... it goes on forever.
You have a couple of options that will help drive your learning and keep you motivated through feeling like a complete beginner all over again:
Good luck!