I'm not sure I've quite followed what you've drawn here, but if all the terrain hexes are on the same sheet, you could use a Color Key effect on that sheet to cut through to whatever sheet you've placed underneath (say one showing the sea bitmap texture). That would avoid the use of multipolys (because it's hard to do anything further with a multipoly once you've converted it to one, if you need to make changes to it, for instance). You could then keep that thicker coastline on its own separate sheet, although you might be able to achieve something similar using a Glow or Outer Glow effect on the sheet with all the hex terrains on too.

However, so long as you're happy with what you've achieved now, I'd stick with that, as it looks pretty good already! It may be worth thinking of changing the river lines colour to match the coastline, so it looks less like the rivers "dead end" there, but that's a pretty minor concern.

For the elevation, you may be better using something like the shaded relief techniques, as discussed recently in this Forum topic. But if you're going with more rocky terrain (or areas of rocky outcrops), symbols probably will work better to convey rough, variable cliff-lines. I'd assumed from the density of contour lines on your base map that the hills were going to be key features, as they're suggestive of something steep and high.

Like most deserts, the Gobi has a variety of terrains, so just pick something that appeals and run with that!

For more realistic drawing styles, and assuming you have plenty of the Cartographer's Annuals available, take a look at the thumbnail images on Sue's topic which shows all the CA issues in one place - here. That should help you start narrowing down to a style you think fits best with what you have in mind. Even if you only have the base DD3 installed, you can find materials to work with there to create what you need, though you may need to be a bit more creative in how you use the various tools and symbols, and may need to draw more, where there aren't enough symbols of the type you need.

Not quite sure about the scale of the map you're emulating, but as you mentioned "tents", I think you're probably right to go with a Dungeon-scale map.

As for the drawing style, that really depends whether you want something a little abstract, but clear - like the drawing you've shown - or something more nearly photo-realistic, or something between the two. Plus what you think looks best among those options, of course!

Another element of this may be exactly how the map will be used, as that could determine how clear you need the various elements to appear - if the slopes are important for movement, for instance, you may need to use variant shading/colouring to make clear for players where there are difficult terrain areas, say (loose sand, hard rock, etc., and perhaps not just on the slopes from the dot-shading in the valley near some of the presumed tents).

Welcome on-board!

The advice from everyone above is very sensible, and should help you get started.

The main problems I found early on with the software related chiefly to being more familiar with how desk-top publishing and graphics manipulation software works, whereas CC3+ is genuine CAD software, which works in a very different way. Equally though, it's also far more powerful and - once you get into its apparent quirks - far more useful.

I found the Joe Sweeney videos the most helpful, as I started with CC3, but the more recent videos (which didn't then exist) for CC3+ by Ralf (the live mapping streams) and Remy are just as useful.

Best advice is once you're familiar with the basics, through video tutorials and/or the Manual, just make some maps of your own, and learn from that - including making mistakes, and working out (or asking here!) how to correct them. Good luck!

Not sci-fi, since they were all done for the Nibirum Community Atlas, which is all fantasy mapping, but they were done in the general style of planetary-system mapping, so may still be of interest. However, I did a series of maps for said Atlas back in 2018 - Forum thread here, Nibirum Solar System start page here, with a sample map to give you an idea, just for the inner planets:

All these maps are also in my Forum Gallery here.