Tried a more subdued colour scheme, and also put together a regional map for the city's immediate surroundings:

So, I thought I was done with this map, but I decided to experiment with something inspired by the amazing maps of Alyssa Faden. I created some new building tools and layers, and applied some slight texturing, to try and mimic the way she does roofs. This allowed me to recolourise the Districts a bit brighter, and to give them (I hope!) a bit more character. Lastly, I labelled it all up with the district and borough names, and highlighted some key buildings.

And here's the final product - thanks for the input!

Here's the last WIP image - I've added every feature I planned to add, and all that's left (I think) is to colour code the buildings by district. Of course, if there's something I've missed, all suggestions gratefully received:

Almost finished with the urban blocks - only the basin to do - then it's on to additional features (city walls, interior fortifications).

Definitely feel like it's starting to take shape...