Ya lo he hecho.
Solo root sin TWRP.
Método:
1) Descargar la imagen parcheada con magisk:
https://drive.google.com/open?id=1DK...0PnHhlGCjioZM_
2) en adb:
PS C:\adb>
adb reboot bootloader
* daemon not running. starting it now on port 5037 *
* daemon started successfully *
PS C:\adb>
fastboot flash boot_a magisk_patched.img
target reported max download size of 805306368 bytes
sending 'boot_a' (26136 KB)...
OKAY [ 0.558s]
writing 'boot_a'...
OKAY [ 0.100s]
finished. total time: 0.664s
PS C:\adb>
fastboot flash boot_b magisk_patched.img
target reported max download size of 805306368 bytes
sending 'boot_b' (26136 KB)...
OKAY [ 0.565s]
writing 'boot_b'...
OKAY [ 0.163s]
finished. total time: 0.734s
PS C:\adb>
fastboot reboot
rebooting...
finished. total time: 0.003s
PS C:\adb>
Luego he actualizado el magisk a la versión posterior.
Excelente por ni siquiera usar TWRP. Lo importante es el root.