The bridge shadows look fine on the ground, but not on the mesa tops, where they look like they're floating again, instead of the bridges attaching to the tops properly

Try moving the bridges Sheet just below the mesa tops one, and then copy the bridges onto a new Sheet above the mesa tops, but which doesn't have any Effects on it. That should still give you the bridge-shaped shadows on the cliffs and ground surface, yet still leaving the bridges looking like they're attached to the mesa tops.

And well done on finally chasing-away those irritating grey patches!

There are quite a few symbols already in DD3 and its add-ons that will work equally well underwater, or which can be repurposed to resemble something of that sort with a bit of imagination. Stone doors will work just as well undersea as on land, for example, while things like fungi groups will work nicely for any colourful creatures underwater - like small sea-anemones, for instance, especially if you make use of the varicolor options. But I'm sure Sue has this well-covered already!

When I needed to add rings for a couple of the planets on the Nibirum Solar System Chart for the Community Atlas, I just used angled lines of a suitable thickness to be seen still, and the line-length correct to-scale for their sizes compared to the planetary discs. As the Cos3 planet symbols with belts in the atmosphere are seen side-on, this seemed a suitable compromise.

It would be possible to go for a more artistic endeavour, showing the rings partly open, but this will occupy more space on the drawing, and I had enough problems just fitting the lines on my effort! It will also make a mess of showing the planetary moons as you have them now, of course.

Do you still have the MERGE Layer (not Sheet) in your map, not hidden or frozen? Effects won't work without it being on and active, as far as I recall.

Otherwise, hopefully someone more technically savvy will charge to your rescue!

Unfortunately Julian, I think you're right. To my eye, ALL these shadowed areas look like they're floating. In addition, the cliff-lines that aren't shadowed, especially those by the lower and right-hand edges look like they're multiple images superimposed on one another, as if out of focus, and that's very distracting too.

For the shadows, I'd suggest reducing their intensity, so they're not as dark, and simplifying them so there's only one shadow per cliff-group. It may be worth changing their length too, though that would need experimentation to be sure.

It might help to add a small, dark Outer Glow at the base of ALL the cliffs, with a smaller Outer Glow at the edge of the stacked cliff-lines that are on top of one another. Sometimes this helps trick the eye into fastening objects down onto the surface more. Again though, this would need some trial and error to see if it helps. Sometimes just a tiny change can make all the difference - it's finding what that is that takes the time though.