The topographic advice here is good if you're aiming for a more-or-less duplicate Earth, as far as range and extent of terrains is concerned.

However, the nature of your initial map suggests that may not be what you'd prefer, so it might be interesting to consider the example of the planet Mars here too, as an alternative. The maximum range between the highest and lowest points on Earth (Everest to deepest ocean trenches) is almost 20 km. For Mars, a much smaller planet than Earth, it's 30 km. On Earth, as mentioned already, there are relatively scarcely any mountains, and no substantial clusters of them, when looked at on a global scale. On Mars, more or less the entire southern hemisphere is rugged to mountainous:

The image above is from Wikimedia Commons, though should you wish, you can find a little more on the topic, with referencing pointers, via the Geography of Mars Wikipedia page as a starting point.

As Sue said, it's less easy finding a decent comparison map for Earth, but this one's a reasonable rough comparison for the Mars one here at least, again via Wikimedia Commons, as found on the Wikipedia Global Relief Model page:

You pays your money and takes your choice!

I appreciate the problems with getting the symbol to work right, Sue. And this one will look good as the tops of forest kelp and other denser seaweeds, aside from its intended eelgrass, so it's win-win, I think!

Mysterious Island keeps coming round on British TV from time to time still. Perhaps most famous still for containing some of Ray Harryhausen's excellent stop-motion animations.

Looks interesting Sue, and it'll work nicely as other types of shallow-water sea vegetation, I think.

I'm not overly familiar with eelgrass, so am having to judge by online images only, but this looks a little large and robust for those types that have come up via Google so far (where there's anything to help judge size, at least, and I'm assuming "your" barrel's about 2-3 feet in diameter). Not sure how representative a sample that may be, however!

Just been doing my usual after-the-event catch-up with this latest video.

Interesting to see the multi-level use of the software being worked through again, especially with techniques many will probably wish to emulate for their own, similar, maps, as well as highlighting some of the complexities of trying to copy large amounts of mapped items from one map to another this way. Things like the addition of new Sheets in the "wrong" order and without their Effects on the new level map are liable to catch-out many on their first trials like this after a clipboard copy & paste, so are definitely worth being more aware of.

This was brought into focus again for me recently with the multi-level Wyvern Citadel maps I prepared, which have just gone into the Community Atlas this evening. I was fortunate with those in one respect, because such maps have to be prepared individually for the Atlas, so I could just save the "old" map and then start the new one from a fresh version of that, which kept the Sheet Effects intact. Of course, even this isn't problem free, because you still have to move around the order of those Sheets that are now at a lower level than the one you're working on, and create new Sheets and their Effects for the fresh items as they're being added!

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!