... the builtin set in Grafx2 pretty much covers it.
The ones that I ever use are highlighted with red. But, unless I'm really laying out a lot of similar patterning -- which I don't find common, in pixel art--, I tend to just go with the 1 pixel brush and the next few sizes
(first, second, and third items in top and bottom row of the large red rectangle.). It just seems a lot less trouble to get the same result with the simple brushes than to make my brain realign to the more unusual brushes ( which tend to be particularly unusual in behaviour when you try to do anything but simply place exactly one of them.
Your image, while each individual rock appears as if it were rendered well, actually demonstrates some of these problems.
When you use a brush that is slanted, it makes it easier to draw things that are slanted that way, and harder to draw things that are slanted in opposition to it. Your lighting reflects this -- you are drawing rocks, but there seem to be a startling lack of sharp angles. I characterize the overall appearance as pearl-like.