You're visiting Mumbai when it's gonna be mostly hot and muggy. And most festivals are some months away. The marathons are in January usually so that's off too.
Monsoon is the best time to visit or just after monsoon -if you don't like getting wet.
I'm still not sure if you're shooting people buildings And in what format - choices of what and where to shoot will depend on that.
Also -are you staying north or south of the city? In proper Mumbai?
Mumbai is an entirely N-S City. Traveling takes time if you're on either end and head to the other - and that might just throw out the options of available light shooting.
If looking to shoot people or sunset a good place is the Juhu beach area. There's areas around Chattrapati Shivaji Station that are populated by century old buildings and cricket grounds, if that interests you.
Gateway is a place that imo smells of the sea too much. But you've got the iconic Taj Hotel opposite it, and there's the Necklace Road.