Screenshots are not a collection
A screenshot loses the timestamps, the participant list, the edit and delete history and the attachments. It also loses the ability to show that nothing else in the thread was removed. A forensic extraction keeps all of it and produces a hash that proves the set has not changed since.
Consent, privacy and personal devices
Personal devices need a documented basis for collection and a narrow scope: date ranges, named participants and defined applications. Collect to that scope, keep the rest sealed, and record who had access to the image and when.
Get the chat into the review platform properly
Chat is only useful in review if the conversation stays readable. Loading messages as individual documents without thread context wastes reviewer time and creates inconsistent coding. Agree the export format before collection, not after.