For sure the cell phone is the best and cheapest device right now. But the cell phone is one of the first things the bad LEO will focus on as well. I have been pondering this for years as well, what type of system should be in a vehicle and inaccessible while streaming video to a protected location.
Perhaps the Dropbox app needs to be a background app so the power-down does not affect and is not noticeable. Only thing else is a wide angle view(s) from of several locations in and around the vehicle to capture most everything happening. No reason Bluetooth can’t be used to do a quick dump to an inaccessible “black box” in the vehicle so save time.
There are very small cameras and car based DVR systems. I’ve seen those and they arent that expensive. The problem is getting the data uploaded in near real time. That requires a cell phone.
My plan is to start recording if an event starts and then hope I can press stop before I lose the phone. Once stop is pushed they would have to turn the phone off or destroy it within a couple minutes to prevent the data transfer.