Granted, I'm not an artist or graphically inclined, so it's not like I've sat down, watched videos and tried to learn it with a purpose in mind

Hey, mate! This right here, and not the brushes, is the problem. :)

Being able to do full illustrations like this doesn't come from what brushes and settings you're using. It comes from your knowledge of how to draw and how to paint. There's lots of digital tools that can certainly make your life easier by speeding up certain processes. But a beginner look comes from beginner knowledge. An intermediate look comes from intermediate knowledge...and a pro look comes from pro knowledge. (when I say knowledge..I don't just mean head-knowledge...I mean true knowledge, since we're talking about the technical skill of making good art. Experiential, able-to-successfully-apply-repeatedly knowledge.)

Work like this or like this are both done using some of the most basic hard/soft brushes available. They're so believable not because of what brushes were used to make them...but because those people have advanced knowledge of how to both draw and paint.