Ok, so now you've got an EDL cable and you know how to make it work. That is one of the problems with trying to make a custom recovery, you can get stuck in recovery. We'll fix that.
First of all, forget that Firehose loader. Use the one for the Page listed here:
https://www.temblast.com/ref/onyxldr.htm. That will get rid of the stupid UFS error.
Now go to EDL mode, load the
new loader. Then make it stop going to recovery.
Now grab the boot image.
Code:
C:\>edl /r /pboot_b boot.img
And reboot.
Install Magisk as you would any other app. Push the boot image to sdcard.
Code:
C:\>adb push boot.img sdcard
Now run Magisk and convert the boot image. ADB pull the Magisk'ed image, go to EDL and flash it to boot_b and reboot. Go to Magisk app and enable "su" for ADB.
Code:
C:\>adb shell
$ su
#
Follow the above instructions for i2cscan and copy the result.