OK, some notes on the space usage:
If you really need your playbacks both in iTunes AND in Stage Traxx, there is no way around having them stored 2 times on your device.
If you need them only in Stage Traxx, you can delete them in iTunes once imported. Or you import via USB device or a cloud service (i.e. Dropbox/iCloud/Google Drive...).
As the backup files also include all your songs, you should leave enough space on your device to create a backup. I will try to add a way to write backups directly to a USB drive in version 3.1 but I am not sure yet that this will work. In the meantime, once you transfered the backup to your computer, you can delete it in the Stage Traxx 3 folder (don't forget to empty the trash after deletion).
There is a bug in version 3.0 that might cause some loss of free space. In certain cases when you add songs from a cloud service, they are placed in folder "Inbox" in the Stage Traxx 3 folder and not deleted after the import. So if you open the Files app, go to Stage Traxx 3 and see a "Inbox" folder, you can safely delete any files in that folder. This bug will be fixed in version 3.1.