Dashcam footage for a delivery van fleet

Your company runs 600 delivery vans, each with a dashcam facing the road. Today the video stays on the camera's memory card. After a crash, someone has to go to the van to get it. The safety team wants footage in the cloud instead.

Each camera records 1080p video at 4 Mbps (megabits a second) while the van runs, about 10 hours a day. Its card holds 128 GB and overwrites the oldest video when full. The camera is powered only while the van runs. Each camera has a 4G data plan of 10 GB a month, and signal is patchy on rural routes. The camera detects harsh braking and impacts, about 4 times per van each day.

The safety team wants each event's clip, from 20 seconds before to 10 seconds after, within a few minutes of the van having signal. A few times a week, after a customer complains, they ask for up to 10 minutes of a van's footage from the last 3 days.

Which video does each camera upload, and when? How does the safety team get footage they ask for later?