For most chats, Unicode text works well, even for reactions. Sometimes the images etc. are important, though. A markdown format might work: it could be read as plaintext or rendered to get images etc. (if you had it linked to the folder of embedded files).
I did suggest MailSQL storage format standard?, which in theory would also work because you could interconvert and view your messages in the threaded view of another mailclient. But obviously export would be much easier to implement!