From a photography standpoint not specific to your equipment, I like to fly in Shutter Priority mode. Open up the ISO to a setting that lets in enough light, but doesn't produce too much noise. You will have to experiment, but this may be 800, 1600, or if your camera is really good, maybe 3200. Slow your shutter to 1/30 and fly slowly. There will be a happy medium between air speed and motion blur. You don't want motion blur. Fly test missions to find a good compromise between ISO (noise) and flight speed/shutter speed (motion blur). Also, the higher you fly, the less your photos will suffer motion blur. So it's all about finding a compromise.
The reason I say to use Shutter Priority is because if you come upon a section that is well lit, the aperture will adjust automatically. This is why I would not use full Manual Mode unless you're sure the entire property is lit exactly the same throughout.
I have a client who does parking lot lighting. I don't don't use mapping for them. Just night photos. These photos were about a year apart.
Before:
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After:
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Here's the "before" photo of the above set pre-processing. I really had to push process the "before" photo, as the parking lot was very, very dark. Getting composition in such a dark area was difficult.
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Here's the same parking lot from a different angle.
Before:
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After:
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I used bracketing for the "after" photos. The hard part is getting exposure consistency between the before and after photos. In this instance, bracketing (HDR) becomes your best friend. As you can see in the second set of photos, the tennis court and building in the background are a bit over-exposed in the "before" photo. But the main building is pretty consistently exposed across both photos. The color temperature is also slightly off. This is the biproduct of shooting when it's very dark with a Mavic Pro and not a lot of experience shooting before/after night photos with that platform.
The reason I show you these photos is because your client may want to opt for this style rather than mapping. On a calm night you can do time exposures, which is pretty necessary to get these kind of photos. You can't do that if the drone is flying a waypoint mission. If they need mapping for geo-referencing certain areas, you can do your mapping during the day. The combination of mapping and composition photos should paint a nice picture and aid with architecture and construction.
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