Dynamesh it. Because the topology of the mesh is the issue. Poles (where 5 or more edges meet) aren't good for sculpting at all. Dynamesh will fix all of that.
Depending on your machine, you'll have a maximum number of polygons/vertices available per subtool. As for the blockiness, it's going to boil down to the topology (wireframe) of the base mesh on the lowest subdivision level; 6 million points might not be enough to sufficiently produce the detail if the majority of those…