What would you think are "sub-skills" of Graphic Design?

Good question. Great graphic design is one of those things like pornography—“difficult to define, but I know it when I see it.” That said, you’re off to a good start listing elements that matter. Add to that a sense of proportion, visually communicating intention, and being able to strike the delicate balance between the comfortably familiar and the provocatively new.

I think a lot of Graphic Design is learned along the way. Design works or it doesn’t. Of course, there is basic knowledge such as the three topics you mentioned, but each can be split up in a fair amount of sub skills as well. Another thing to remember is that on one hand there are rules, but on the other hand there aren’t. They’re more like guidelines anyway. All matter is fairly subjective: you might like what you created, your best friend might not.

Trends are important to keep track of, and as @ETpro said before, you need to find that delicate balance.

One tip I can give you is to start hoarding any design you like. (Pocket is a great app for this)
Not just graphical design, but industrial and web design as well. Certain subreddits are a reliable source.

Then try to recreate these things, or create something new from them. Share your work and ask for constructive criticism.

I think what separates graphic design from art is that graphic design has a purpose, a message, and a target audience. It is often used by people.

With that in mind, I think two skills that often get overlooked are communication and usability. A design might look great, but if your copy is full of gramatical errors and misspellings, it will get in the way of communicating. A design might look great, but might be difficult to use.

Balance/symmetry (more than just layout) is a huge part of design.

You got it more or less right.

I think I’m going to break it down a bit, and start with logo design – not to say that it isn’t part of the broader design spectrum, but at least in narrows it down a bit. I’ll still use all the principles above, but I think learning all of graphic design, “fast”, is a losing battle. Thanks all – I’ll keep everything in mind!