Really strange..I'm out of ideas. I followed your guide exactly.
Flashed 18.104.22.168 OTA.zip
The log said "installing to inactive slot _b, reboot recovery to flash more zips"
I wiped dalvik,
I rebooted into recovery slot b (I didnt boot into system)
I installed magisk 23.0 APK renamed to zip. It installed into slot b.
I did not wipe dalvik.
I rebooted into system.
I still get a stuck boot (OnePlus circles rotating)
Bonus: after "fastboot flash boot stock_boot.img" it boots fine (but no root)
I never installed TWRP, just "fastboot boot TWRP"
I understand nothing
Last thing I can think of other than a clean start is to flash magisk uninstall.zip, maybe there's leftovers from broken module or something.
It's a long shot but worth a shot, other than that sorry I have no idea what's wrong here.