Now Magisk Canary (19303) has fixed the boot loop issue.
Everyone can use the newest Magisk manager, select "Canary Channel" to patch the boot.img.
First , unlock your Xperia 1.
to unlock your Xperia 1.
If you can not find the Xperia 1, you can choose Xperia xz2.
It also give you the right unlock number.
Second, patch your boot.img
the newest Magisk 19.4 is not supported for Xperia 1.
You must use the Magisk 19.0 .
downlowd the rom and unpack it, find the "boot_X-FLASH-ALL-A2CD.sin", it's the xperia 1's boot.img.
downlowd the magisk manager, install and open it.
Go to settings, tap the "Update channel", and select "Custom", then type it with this
Go back to the Magisk manager, tap the "INSTALL", and "select and patch a file", select the "boot_X-FLASH-ALL-A2CD.sin", wait a moment.
then you will get a patch_boot.img.
Go to Fastboot mode, and flash it. "Fastboot flash boot xxx.img"
I will put my patched boot.img here, but my xperia1 mode is J9110,CHINA MODEL. it may be not supported for you.