Those are all important things to familiarize oneself with, but let me answer your question this way.
Let’s say I wanted to be a professional basketball player, so I study all the rules. I look into all the right clothes. Get just the right shoes. Learn all about the physical characteristics of basketballs, hoops, backboards and court floors. I study the NBA. Learn the names of all the teams and the leading players on each. I take up running to get myself in the best possible shape. I do everything I can to learn about basketball. The trouble is, I’ve forgotten something. Despite all of this, I still can’t play pro basketball because I lack talent and have nowhere near the hours of needed practice and experience. Without that, I can’t really do much with any of those tools or the knowledge I’ve learned — the knowledge might be there, but the core ability isn’t.
I could have used becoming a rock star as an example, or a race car driver, or an airline pilot, or a sculptor. Everyone learns these things by starting at the beginning and putting in thousands of hours of practice and guidance from those who know more.
As important as learning everything you mentioned might or might not be, those are just odds and ends, the tools, and the practical knowledge that it takes to implement what all those thousands of hours of guided practice prepares one to do. Without the talent, skill, and experience, it’s all just academic knowledge that doesn’t add up to a good designer.
That’s an important observation and a considerable downside to teaching oneself — the teacher knows no more about the subject than the student. There’s a good deal of practical knowledge required, and your post indicates that you understand that. However, your post doesn’t show a realization that proficiency comes from practice, guidance, failure, criticism, struggle, dedication, and endless hours of all those things. You can’t learn to be a basketball player, a musician, a race car driver, or a sculptor from a book or from searching the internet.
As you’ve likely surmised, I’m not a big fan of trying to teach oneself graphic design. Not only is it a field wholly oversaturated with beginners, but it’s also a field where the starting to intermediate salaries are heading in the wrong direction. In addition, it’s a field where you’ll be in head-to-head competition with those designers who graduated from university design programs and interned at various agencies and businesses where they gained insight and experience from those more knowledgeable than themselves.
If what I’ve said hasn’t dissuaded you, and if you’re really committed to packaging design without spending four years in school, your viable options are limited. Still, others have done it. Try looking for a job where you can start at the bottom and learn the skills you’ll need and where you can develop whatever talent you might have. Honestly, though, I’m not sure where you’ll find that job unless your portfolio is just oozing with raw talent.