It seems there is a problem regarding ham and the encryption that simply refuses to activate. The phone reboots and immediately reboots back.
Maybe it's related to the TWRP 3.2.3 unofficial version, the last one available for ZUK Z1. And both official LineageOS 16 and LineageOS for microG keep it, while on some other phones it's systematically replaced with its own bootloader that allows the encryption.
Because for me an unencrypted phone doesn't exist, I can't go any further so far. Any idea about the installation method with LineageOS own bootloader because TWRP 3.3.1 that manages the encryption well isn't available for ZUK Z1.