Have the car store its own data. You can fit 500GB on a micro SD card, think of the storage you could fit in an entire car.
Lower the framrate. 1080p at 60fps, but anything above 30 looks smooth, and you can go all the way down to 12-14 frames and still have pretty good video.
Run local event detection on the car, and only have it upload small segments of video when it detects certain events.
Allow a control device to request video that are stored on the car the next time the car checks in.
I think limiting the data collection in this way would allow full surveillance when desired, but not require a lot of overhead on the network.
Ultimately, the "limiting factor" for these kinds of systems is the human element. You can only hire so many people to review so much video in a given time period. AI is changing this, but even then, you can only hire so many people to review events flagged by AI in a given time period too.