I bet you could fix the misidentification of ridgelines, by coloring them brown - their present color looks navy-black, hence distinctly riverish. Or maybe dotting them, like border lines sometimes are done - that'd make the obviously symbolic rather than tangible.

What's the significance of the two different blues in river coloring?

If I'm guessing the scale right, your rivers have some seriously implausible dividings and rejoinings. Islands along rivers tend to me modest sized; some of these seem huge. Basically, water chooses the single lowest course in any situation. Why would it ever want to flow down a less-low course, when a more low one is available? Small diversions around what you could think of as a mid-river hill *are* plausible, but that's only the smallest of them. The deeply-incised bevel you've got going with the rivers actually worsens this implausible-river problem -- one does get meanders and threading of rivercourses in dead-flat terrain, like river deltas, but your standard river 'look' has some serious banks going on, like mid-life rivers have. Outright *lakes* have essentially flat surfaces, so they could accumulate a collection of islands, but that's not what you seem to be going for.

Most of your river joins are at about ninety degrees. Without more sharper-angle connections, there aren't many cues as to which way is downstream. I honestly can't tell if the main river network on C4 drains off the south of the sheet by that ruined keep, or off the east edge.

What's your thinking on the ridgelines getting their own shadows? It makes them look like great-walls-of-china instead of lines drawn by the cartographer on paper (or screen). I actually like them as indicators of ridge networks, I just wonder if they'd work better with less emphasis. Those cousins to ridgelines you've got defining canyons do help make the drop-off obvious, but do they also need their own shadow or glow? It again makes them look like 3-d 'things' sitting there.

Is there a reason some ridges don't get ridgelines? Like the SE N-to-S one on B5. It looks just as inhospitable as the S and SW ones there.

Your overall effect is great, and if you've got techniques for generating it without a gazillion hours per sheet, your project will not only prosper but be a really amazing collection of maps!