Advice on the index for a city map

[ReverendRover]

Hello all.

I'm working on a campaign setting for a new D&D campaign based mostly within a single city with occasional jaunts out into the surrounding kingdom.

I've got a (mostly) complete map, and a good idea of the major districts etc. but I want to create a very detailed key showing each district, then breaking down what is contained within. It's going to be quite sandboxy and driven a lot by the player characters connections (friends, family, patrons, rival gangs etc etc) so I need to get a lot of detail in without overcrowding the map or having the key to it be a horribly long biography.

Any of you with experience doing large cities have any suggestions on how to organize such a thing to make it both quick to reference and still keep the map as clutter-free as possible?

For reference, here's my working version of the map