1. run full phone backup on mac
2. get latest version of imessage-exporter – https://github.com/ReagentX/imessage-exporter
3. get imessage-exporter running (chmod +x imessag-exporter)
4. run the following command ./imessage-exporter -f html –platform iOS –db-path /Users/USERNAME/Library/Application\ Support/MobileSync/Backup/BACKUPID -c compatible
   • this will copy all messages from the phone and attachment images to /Users/USERNAME/imessage-exporter and the images will be in /attachments
     

reddit post with more info — https://www.reddit.com/r/MacOS/comments/14jy8h7/how_to_export_whole_imessage_conversation_lasting/