You could try drawing simple shapes (lines, circles, triangles, say) with the Solid White bitmap fills, and experiment with blur and/or edge fade effects on them, rather than looking/hoping for actual symbols. Or just use solid white/grey polygon shapes, with transparency, blur or edge fade effects on those instead - or indeed a mix of all. You may need a few extra sheets, but you could get a more unique look that way.

They're mostly the varicolor rock symbols, with a colour chosen to more or less match the ordinary hill symbols, though I did rescale some of the normal hills in places as well. The rock symbols have little dots with some, which helps make the terrain look a little vegetated as well, which with the bushes, fills things out really well for just a few clicks here and there (because the symbols cycle through the options randomly if you don't expand all the catalogues).

I've never used FT3, so can't advise on specifics regarding that. However, from seeing how others have used it over the years, and how its use is described on the ProFantasy website, one option beyond Sue's suggestions that occurs to me would be to randomly generate one or more FT3 worlds, changing each one (remembering to save any that seem interesting first), till you arrive at one you feel you could fit your extant mapped areas into, and use that as the basis to work from.

As Sue said though, you will need to do most of the work you're wanting after that by hand - the blending, adding undersea features like coral reefs, etc., so this might not be any quicker than doing it all by hand anyway.

Having performed the usual New Dungeon Map Ritual, in consultation with the CC3+ Wizard of the New Drawings, and imported the hand-sketch for the map to a fresh Bitmap Sheet and Layer, as normal, the first section of the subterranean level could be prepared, essentially amplifying the Below-Ground Level of the Quezzal Tower map:

As we see, I'm using Ralf's Hand-Drawn Dungeon style again here, and it's likely also obvious to those more familiar with the style, that there are a couple of cavern walls missing here. These are where the next areas connect to this one, connections which are somewhat complicated by the fact the link-areas are actually cliff-lines in the cave, indicating steep rises and falls in the level. This part of the drawing already shows what happens when floors are all drawn on the one Floors Sheet, which has a nice inner glow effect on it, that the walls increase with their own glows. All the floors flow straight into one another, without any such shading effects. That's great if all the floors are on the same level, but cliff-lines mean they aren't. Thus, enter some new Floors Sheets:

Only it's not quite that simple, because to fool the eye properly, the floor level above the drop-line needs to be lighter up to its edge than the one below. Thus the floor level apparently above the drop line has to be drawn a little longer there, on a Sheet BELOW that on the other side of the drop-line. So the way I had the floor edges on the initial snapshot image here was only correct for the broader of the two connection points. The narrower passage one had to be edited after the fact to extend under the next upper, though actually lower in Sheet order, floor. And as the second shot above indicates, there were sometimes going to be multiple levels on the same passageway...

This wasn't actually quite so bad as it may sound, because I'd already discovered how to get round the problem when drawing the map for the Tomb of General Chengdai previously. Although I didn't draw attention to it there, nearly all the adjacent floor areas of that dungeon map are on different Floors Sheets as well, and the sliding walls have had an additional external glow effect added to their Sheet to help maintain a similar darkening close to them too.

Even so, it needed a couple more sessions to get the whole layout drawn, and I had to add an extra Sheet at one point because, for entirely unfathomable reasons, one of the new Floors Sheets started causing transparency acne problems with the next Floors Sheet in the stack (but none of the others did so, despite all being identical, and having minor overlaps in places, nor did the problematic Sheet with others that weren't adjacent in the stack...):

The freestanding rock stacks were drawn using the Cave, Cut-out drawing tool, that makes use of the Color Key effect to punch through the grey stone floors to show the brown stone background, and make it seem like they're upstanding features within the caverns. Occasional tweaks were needed to get rid of a few too-sharp fractal points along the edges, taken-out individually rather than using the "Simplify" command, which I find sometimes takes away a bit too much on this kind of smaller map.

One element that slowed things a little more was adding the cave walls after various segments of floor were completed. "Trace" does help hugely in this, although the individual floor patches meant constant chopping and changing as to which part was being traced at times. So this didn't leave a lot of time to start adding the first interior details:

Just some debris and the floor circle in the Old Summoning Chamber, the pool in the next main cave along (yes, that's going to need its own pool-edge drop-line at some stage!), a large central-cavern chasm, and, ooh, glowing spiderwebs! What can those mean? More next time...