Is there a reason we would need to coordinate on what to exchange rather than, say, beginning with encyclopedias and textbooks then moving to a constant stream of notable papers, news, discoveries, etc? What kind of bandwidth can you hit with a cooperating neighbour where improvements become civilisationally important? How many bytes (megabytes? Terabytes?) of meaningful new data does humanity produce per second? I suspect it's reasonably low.

Good question. My thought is similar to yours, but there is a lot of room for debate on what to send. They probably don't care about the roman empire like we do - but there are enough references in modern science that we need to send a summary just so they understand some things. We produce a lot of data, but most of it isn't meaningful.

if this is a real situation I wouldn't be asked. So use salt