As an alternative, make a local TWRP backup and use the "adb push" command to send the backup to your computer for external storage and then delete the local backup on your tablet. If you are unfamiliar with using adb, you can use Mark Skippen's toolkit to safely transfer files to and from your tablet. I advise against using the Windows file manager to move the files, because the tablet connects to a Windows computer by MTP which is unreliable for large file transfer.
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