It may be worth noting that using the "C" = corner command to add extra nodes can sometimes create problems because of the extra nodes very close together this generates, and you can end up with unwanted extra lines you didn't create apparently linking unconnected parts of the line to one another. You may only see these at certain zoom levels when viewing the CC3+ map, but if you can see them, they'll often export if you try to save an image, or print it out. This is far more likely to happen with fractal lines or polygons than smooth ones, as they have far more nodes in general, but that "C" command isn't always problem-free either, unfortunately.

I don't think there are any symbols that will do this for you easily, so you'll need to draw most of it. Thus you can just pick whatever style seems best to you for the look you're wanting to achieve.

As your initial drawing shows though, even a simple pit will give you a template from which to work with for this, although it may help to pick a spiked pit to assist in getting the descending pillar polygons to look right at their various angles. The wall lines alone help with those near the pit's edges, after which you simply need to adjust the angling as you work inwards. There's a spiked pit in the standard DD3 symbols, for instance. To make it rectangular, simply set different suitably-sized X and Y coordinates when you're adjusting the symbol's size. This will produce distortion (especially of the spikes), but as you're just using the symbol as a guide for drawing to, that shouldn't matter much.

The pillar polygons will need some shading applied to make it look like they're fading away into darkness (like the pit walls in your post here), which can probably be best achieved using additional polygons drawn over the "lower" parts of the pillars (try the darker Solid bitmap fills that have various transparencies built in), coupled with some edge fade effects to blend one to the next (you'll need to set up several extra sheets to get this to work well enough). It probably won't be quick or easy to do this, because you'll have to keep stopping and thinking about what to do next (I know I would, which is what I'm basing this on!).

If you get stuck, just ask again here. Or maybe someone else more technically adept will come up with a much more elegant, and swifter, solution in the interim!

Still reeling from the shock that the style wasn't called "Sinister Suers", naturally 😉!

If you're using the Outer Glow effect for the text, you might want to try just using Glow instead, as that sometimes seems to be less "misty" when used with text. Or you could try with a pale glow around some of the lettering, not a dark one. Larger text should help, certainly, but you may need to use different sheets for the different text sizes so you can adjust the effects more easily (and also perhaps separate sheets for the darker and lighter text colours).

Have you thought about leaving the hatching off entirely? You really only need it if there's nothing else to show what's solid and what's not, I suspect.