I have kept black cars and gun metallic as well. I had been using mothers synwax before but after that was not available I got 3m show car wax ( https://www.3m.com/3M/en_US/p/d/b40066677/ ) .... the results are amazing especially on black and dark colors. Both waxes mentioned once a month or maybe 3 months ( not remember exactly ) but i used them like after every 2 - 3 weeks. That made a good layer of wax on my black car.
To clean the car although i dont do it myself now but i have trained a guy at home for it. IF there is too much dust you have to blow it off first maybe use a blower or just use a cloth for it. Once most of dust is removed, use slightly wet microfiber. Do not clean it the typical way our drivers at home used to do which puts swirl marks. Open the cloth and just spread it on the car and pull it back. No hand pressure only the weight of cloth cleans the car. That will save the swirls to max. On sides also i just dont put hand pressure at all. Move the slightly wet cloth just like its hardly touching the car.
Any wet/dry mud on the sides, then better wash it. Never try to remove it with cloth. Similarly if there is bird poop on the car, I just put a wet cloth on it for some time to make sure it removes off itself. Any residue sticking to car sometimes from plants, do not rub it. Either shampoo wash or put wet cloth on it.