Hello @Remi,
you can do the NanDroid backup and the /e/ OS ROM installation with the TWRP already on the device.
I recommend to save the NanDroid backup directly to a microSD card. From the microSD card you can also do the 9-pie upgrade.
For the NanDroid backup boot into TWRP Recovery …
TWRP Recovery :
With the device powered off,
hold Volume Up + Home + Power.
When the blue text appears, release the buttons.
… and tap Backup there and intuitively follow the offered commands.
This guide will help you to manage the /e/ OS upgrade…