actually there are these two challenges: overheating shortly before or after the start and later the extremely low temperatures.
I recommend from our experience of the last 10 years and about 1000 stratospheric flights that everything should be made ready for takeoff shortly before the launch of your weather balloon: Fill and seal the weather balloon, install and test the GPS tracker, install measuring instruments if necessary. So prepare your probe and weather balloon and parachute completely and only as one of the last steps switch on and install the camera. Then the danger of overheating is low. Furthermore there should be a little space around the camera in your styrofoam probe, so that the heat is dissipated. We strongly advise against using a pocket warmer, if a polystyrene probe is used. This leads in almost all cases to overheating!!!
There are never problems with the cold if a polystyrene probe is used. We personally use the Space Cam Apeman for our flights or other action camps, except GoPros, because especially the newest models almost always cause problems.
I hope that I could help you a little bit.
Here is a link to our tutorial, there you will find more useful tips and tricks and hints as well as a free app that will surely help you on your mission.