I have attempted several times to use the Rekordbox 6 DB import to some success but with some symptoms.

It appears that after uploading (+1600 songs) and patiently waiting for all the uploading and synchronizing to complete, that some playlists contain songs that are entirely blank. No name, title, artists, bpm, stars, or anything; completely blank. Some songs within the same playlist do show up but are not clickable or useable in the phone app.

The Fix: I deleted all the playlists and uploaded the XML using the Rekordbox import (not the 6). It prompted me that it would not upload the following songs (all songs) and that the playlists were made with the songs. Now there are no more blanks and all playlists are correctly associated.

Feel free to contact me for more details if necessary.