I can answer a couple of your questions and may revise as I learn more.
If you publish something then in many places you have copyright already. Registration of the work may be required in some places and once was in the US, but no longer. Here, at least, if you publish something the copyright is yours and while your words can't be used without a license, your ideas can.
The way to protect "inventions" is through patent. This will almost always require registration (and cost), but is the only way to keep others from exploiting your ideas (as distinct from your words). But if I describe an idea that leads to a "process" or invention, but don't patent it, then others can exploit it freely. They might even try to patent it themselves, but you can (more money) probably prevent that.
One of the big problems for a researcher is gaining a reputation. This implies the need for your work to be seen by as many people as possible. Self publishing and writing a journal has the problem that it is difficult for people to find your work. Publishers, with all their problems, provide a partial cure for that, as do conference presentations.
Finally, for now, you are unlikely, in the absence of a patent and an implementation of the patent, to make very much money from publishing. The exceptions are textbooks for elementary economics or calculus courses, but only a few others. If you want to make money from publishing, write popular novels. (But "popular" is the hard part.)
Disclaimer: I'm not a lawyer and the law varies widely around the world.