I tackled something like this a few years ago, it's only hard if you try to depict everything. I'd still go for top-down view, probably with additional vertical profile views to show the heights and the connections. At least, that is what I did.
Also, to get a rainy look I would try to highlight the consequences of much rain - big sewage, drain pipes, rivers forming in the city (!), dams to protect specific areas, huge dams with a sea to generate water power, etc. You should also try to avoid cast shadows ant the like, because you probably have a very clouded sky. Which leads to probably bombastic looking light coming off of some reactors, power plants, bigger buildings, etc. You should also think about how you feed all those people without any sun and constant water. Maybe some algae-stuff XD
Sure, it's medieval, but there is a city-planet called Ravnica (Magic TG-Universe), just google it for more information. Probably you get some inspiration there. Even Dave Allsop worked on this
Cheers, Steffen