Here is a prerooted boot and system twrp backup all you will need to do is install supersu from the playstore.
1. Backup something in twrp so the folder structure is generated
2. Then unzip file to /storage/emulated/0/TWRP/BACKUPS/NBQGLDXXXXXXX (This may change depending on the phone idk)/
3. Now go back to twrp and click restore and select the backup
If you never set a encryption password (It asks you during the startup wizard), then you just click enter and skip it.