It looks like your symbol set is transparent so where they overlap you can see through them. You've done a pretty good job at avoiding such overlap, but it still shows up in a few places, particularly the forests on the islands. Adding an opaque background to your symbols would make things a lot easier, and give you more flexibility in placement.
The effect along the coastline also has an artificially precise look that clashes with the rest of the image. Drawing very precise parallel lines very close together like that is hard. You should also be careful about masking out wide washes of colour too as that tends to require techniques like wax-resist which are more common with textiles and which as I understand it tend not to be suitable for really fine and even lines. It's the kind of thing that can give a map a really obvious "done on a computer" look. If you want to do something about it, I'd suggest increasing in spacing between the outset lines and using a vector outset operation rather than a raster one if you have that available (Inkscape can do it if you don't mind using another program). Then maybe add a little bit of jitter or some natural noise of some sort if it still looks a bit too precise.
If you want to emphasize a "hand drawn" look, try to think about the media and process behind it. For instance, a coastline that's purely defined by a colour wash would be unlikely as a colour wash would be added toward the end of the process for emphasis and decoration once all the features had been put in by some more precise means like a pen. Little touches like that aid in suspension of disbelief for the viewer. That's why for maps like this I generally choose to get all of the "pen" work in first and then add textures, and whatnot afterwards, if I do so at all.
Thanks for your response. A lot to think about; luckily I'm not that far in that the changes can't be implemented.
I've never figured out how to fit a background to my symbols but I'll do a little Googling now that I know it's possible.
I'll have a look at fiddling with the coast line. There were a couple of things I didn't understand (vector outset operation for example, lol).
Thanks for the feedback,