Backing up a phone and keeping a conversation are different problems, and this solves the second one: pick one thread and it writes out a searchable, offline, iMessage-shaped website with both sides of it, plus the photos, videos and voice notes stored on that Mac. It reads the Messages library without touching it, and ships JSON and Markdown transcripts and a checksum manifest so the archive outlives the app. The free version exports the last seven days, so you can see the output before paying.