Well I mean you could rig the cameras to turn on when the cop gets out of their car to break the footage into specific encounters where the cop had to interact with someone. Identify the files by the date, time, and badge number of the cop the camera is assigned to, and now you've got an easy to search database of footage whenever an incident is reported either by the cop because they had to issue paperwork for it or by whoever they were interacting with because they want to lodge a complaint.

While randomly selecting files not involved in ongoing investigation as potential training material could be helpful, we don't actually HAVE to have an assigned review resource to scan for bad behaviour or relevant material to investigations since in both cases someone is incentivized to start the process that will pull the relevant footage anyways.