I wasn't totally satisfied with the layered fractals being the stepping-off point for a true material density map, so I made some revisions to the code. I spent several days delving into various noise functions and eventually came up with a fairly fast and simple method of blending the disparate-looking fractals using a sort of hybrid Perlin-noise-with-cosine-interpolation function. These are a few of the results:
A lot closer to what I'm looking for. The blending functions aren't perfect and still leave some of the undesired "webbing" between less dense parts of the image, but I can start from here. After basically spending the entire week playing around and with and digging into various noise-generation methods (Perlin, Simplex, midpoint-displacement), I've got a handle on to how to use them going forward for a lot of procedural stuff. Time well spent.