If it were me I think I would merge the files on the pc, save them to a new file, then re-install [batch import] the merged files to MS. A PITA but probably the best way to get to where you want since the time requirement is going to be similar. I merged mine [all pdf's] before importing.
Dell Latitude 13.5" 2-in-1 Ubuntu/Win 11
Samsung Note Pro SM-P900 12.2 Android 5.0.2
Samsung S7+, Android 12
Samsung Note Pro SM-P900 12.2 Android 5.0.2
Samsung S7+, Android 12