Learning system development. Period. Heck, I'm still looking for resources that don't cost hundreds of dollars, require me to open-source everything I want to make, require that I read a 4000-page Intel manual, and/or ask that I be in a graduate program or have a degree that I am still earning.

What do you wanna do? There are trillions of tutorials on linux system development.

Playing around with driver or kernel development doesn't cost you anything if you're doing it on your computer or in a VM.

If you want to go down to the bare metal, buy a raspi or arduino for a few bucks and get it going. For those you will get nearly any documentation for free. You have to read the documentation, of course, and it usually consists of more than a few pages, but learning means a bit of struggle.
And a large part of raspi/arduino documentation is suitable for beginner.

Just start and see where it leads you.

You're right. One of the major problems I face is the fact that I'm trying to go too far too fast. I'm also obsessed with wanting proprietorship over what I make, and being able to do such things as writing libraries or a kernel. It's clear to me now that it's not very possible right now. I'll see where this route takes me, hoping I can in my busy life.

Just start with the typical Hello-World tutorial. There are plenty of basic things to learn before you can be "productive".

You will not implement a video driver for an Nvidia-chipset on the first try. But if you do, I will really envy you :-)

I suggest a raspi, there are tons of simple driver development tutorials out there for it. And if it annoys you some day you can still use it for other projects.

And for the licensing part: As long as you do it for your personal amusement, you don't have to disclose anything.
And even if you're selling it, it doesn't necessarly mean you have to hand out your full source. That depends on the exact license and the way you're using or changing OSS software and libraries. That's a really tricky subject.