People sometimes use sand bags to hold device mountings to the dash when they can't use the windshield.
Some people make their own weighted bags at home using sand or pebbles or other heavy items.
I have suction mounts for the window, but only need two, one for the accident camera in the middle of the window, which I rotate around to look out the back window when I'm towing, and the other one on the drivers left holds my android phone for GPS. So can't really help with your other devices as haven't had that problem with so many devices.
As far as theft goes, the camera is easily removed from the mount and stored under the seat so it's out of sight and easily returned to the mount when driving. The phone always goes with me whenever I'm out of the car.