Soft and hard round brushes can definitely work, but they tend to give you “fluffy and transparent” results unless you’re very careful about controlling your gradients and opaque areas. The most important thing is probably to punch contours by looking for any “brush spray” where you don’t really want a soft transition, and then going in manually on either side of the edge and enforcing the colors until you have a pretty solid border. The soft round brush will always shoot for the fluffy side, so you need to fight it by actively trying to make harder features.
Also, where you want a fairly uniform area you need to colorpick a nice representative color from the overlaid hard/soft strokes and repaint it with a soft brush to cover all the small transparent marks from the hard brush. It can be reasonably quick to doctor an image up this way and it’ll look a lot more solid, but ultimately there’s almost no chance of escaping a somewhat sterile digital look unless you go for textured brushes. You might want to look for a couple of brushes that feel similar to your current round ones but have a bit of grime or bristle in them. Perhaps plain brushes have an educational value though, in that they are extremely deterministic and won’t distract you with spurious patterns and detail. Easier to focus on pure painting skills.
Oh. One interesting trick is to start a new temporary layer and paint a region there with solid color, then adding a layer mask and tweaking or shaping it with black/white and the opacity slider before you flatten down again. That way you can paint something like a tint, light or shadow with much more strength and freedom, before softening it and deciding the exact effect related to the overall balance of the picture. This can be done regardless of what brushes you use of course.
Edit: I realized this is actually just advice on making your current tools work better for painting, not really anything that will help a lot with quick sketching. Personally I like to search for ideas by scribbling large (or small) shapes very loosely and looking for interesting things that jump out at me, given a current mindset/theme. You might also want to focus on very strong simple lighting schemes if you’re going for a painted look, since that can let you drop a lot of the scene into thick shadow which saves you work, forces you to think about fundamentals and often makes the result more attractive too.
