Structure markers - yes, some probably are too large. You might need to replace the hollow centre to the Town and City ones with a contrasting colour circle/diamond instead of being hollow, if reducing the sizes (looking at the way the smaller markers are on the map now).

The craters look nice, although the pale outer rim is a little too sharp and dominant at present. Crater rims tend to end up as a relatively gentle hummock-line very quickly, through a mix of natural slumping during to soon after formation, and weathering processes. Currently, the rims look a bit too wall-like.

They all seem perfectly fine to me, Sue. Natural terrain colours don't always match quite how you'd expect, and can change under variant lighting conditions anyway.

I can also see that "patchy" texture look working well for different kinds of swampy lands, with a suitable water texture showing through between the islands.

Late to the party, but lowland tundra is essentially cold, dry and treeless. In general, it's the name used for such treeless places where the ground is permafrozen a short way below the surface, so shallow lakes and bogs are common, when the topsoil's frost thaws in summer long enough for hardy, low-growing plants to survive. Alpine or mountain tundra is similar, but its treelessness is because of the poor, thin soils, and colder, higher altitude air, as well as general dryness. Lakes and bogs are less common, as the drainage is often better in the higher mountains. It's all downhill from there, after all 😉.

Red desert sands are fine by me too - handy for anyone wanting to recreate Martian landscapes, of course!

If you wanted to add some rails and railcars, Remy did a livestream video on drawing rails and railway rolling-stock here, which may be of use.

It's just a thought, but at this large-scale of map, you can use the straight drawing tool and get a nearly identical "natural" appearance, simply by clicking more nodes as you go along. Although this obviously creates more nodes, they'll end up as a mere fraction of what you'd get by always drawing using the fractal tools, which often add far too many nodes on their default settings. You'll also end up with a map that looks pretty well identical to this, but one that you'll have more control over in all manner of ways.