Sorry for double post but although it was successfully installed it appears that TWRP can't find the partitions.
It only shows a "internal storage" with 5 Mb and nothing else. I can't mount any other and on the backup menu there's only system, cache and data, all with 0 bytes.
I'm going to try other methods to see they work. If all fail I'll have to try to install Magisk without a custom recovery.