Before really going deep into networking, OSes, distributed systems, perf and scalability, I'd say "learn monitoring, alerting, and dashboards, and then learn whatever your data indicates as issues."
Do that right and you'll have a much directed path towards what to learn next.
Logging, monitoring, etc... is of no value if you do not have an intuition of what to monitor.
Monitoring is mostly reactive, to stay afloat you should focus on prevention: know your code, keep it clean, test it, do load testing... and monitor it.
All of these answers are too simplistic. There's no single answer in what to learn to become a software engineer. If you have no development background, learning javascript, node, react is a way to start down the path then build your skills as you progress.
Personally, I recommend learning domain modeling and understanding entity relationships, primary and foreign keys then SQL. Great foundation for engineers at most companies.
That is good enough if you are going to be supervised by a senior developer that can mentor you, but is still not enough to be an autonomous developer.
Do that right and you'll have a much directed path towards what to learn next.