TL;DR;
The basics you should know, IMO, are:
- Write and solve algorithms. This is the base of everything else.
- Object oriented programming. I’d dare to say that every programmer understands this methodology, but you can choose the next one instead (or both).
- Functional programming.
- Architecture patterns. MVC, MVVC, REST and microservices (they can be combined) are the most common.
- Learn one front end framework (React, Angular, Vue, etc.).
- Learn one back end framework. Varies depending on the language (Node=Express/mongoose/passport, Java/Kotlin=Spring Framework, .NET)
- Security. This is something that one should specialize, not be a generalist (one man army), IMO.
How does one know one is ready if one doesn’t even know what one needs to know to be ready?
If you build a software and it works, then you’re ready. Yes, there will be flaws, but then no software is perfect (there is this thing called the human factor, know it? 
).
‘building a system’. What do you need for that?
It depends on the system you’re trying to build. The system may be an O.S., a Platform as a Service, Software as a Service, etc., and each require different skills. On the other hand, technology is constantly changing, but the only thing that won’t change (at least until quantum computing is more mature, but keep an eye out) is how to program a software. If you learn how to program, you can learn any language (some may take more time than others).
I understand and I feel the same. The problem is that you want to be a one man army, which is hard and requires a lot of knowledge, whereas if you had a team that knowledge would be shared (logically, no? ). I’m just like that 
, you just have to be good at learning and be patient. Choose whatever technology you enjoy and stick with it.
I would say that, just for what you want, FCC does an excellent job with the slogan Read-Search-Ask. This is because it forces you to search and learn what each concept means, which should lead you to more search and more understanding of the system you’re tasked to build. When you’re a one army man, you have to do research, a lot!
That’s all you need 
. I would add, though, to learn about security or find someone to help you find the holes. If your software is open source, you could have thousands of people auditing it .
By the way, you cannot feel embarrassed if you’re yellow duck .