That’s just the nature of the grab tool. You can try to mess with the curve settings of the brush where you may get slightly better results, but it will never be able to move your flattened surfaces perfectly.
Alternatively you can hop into edit mode, select the faces of your flattened top, enable proportional editing and make your adjustment that way. It could be potentially slow depending on how many polys are in your mesh, but it would also maintain the exact shape of your selected faces.
As for critiquing your progress so far, it’s looking good. It’s looking very stylized and kinda gem-like since it’s entirely faceted. If you’re going for more of a boulder, crags and cracks will help.
