I drive a pick-up truck with multiple cameras that I often leave running while parked using two power banks. On extremely hot and sunny days I won't run the cameras at all while parked but most of the time I do and have been doing so for years now with few if any issues.
Sometimes, if I want to run the cameras and am concerned that my midnight blue Tacoma might be getting too hot inside especially on brutal summer days, I'll crack the windows a few inches, but not so much that anyone could fit their hand or arm inside the cab. I also sometimes open or partially open the rear slider window in the back of the cab. It helps that I live in a relatively low crime area but it's not like we don't experience criminal behavior, especially with the opioid crisis these days. Nobody has ever messed with my vehicle and the cameras would capture them if did and someone would have to smash a window in broad daylight to gain entry which I believe is unlikely in my neck of the woods. I do take into consideration how long I need to leave the vehicle parked unattended. On a hot day I might leave the cameras running for a few hours but I probably wouldn't leave them running for an entire day sitting in the hot sun.
I think it is best not to use a sun reflector if you want to run dash cams in parking mode, even if you cut a hole in it. This is a topic that has come up quite a number of times on this forum and the consensus agrees with me that it's not a good idea to mix dash cams and sun reflectors as it is likely asking for trouble really.