So you can boot your phone after flashing Magisk patched image from fastboot? You wrote above that system would not boot after flashing..What I mean is that you can just patch your boot.img in magisk, go into bootloader and straight up go "fastboot flash boot <patched image>.img" and reboot and that's it. I don't know about the recommended way, but doing it like that works just fine after flashing F.18.
P.S. Flashing boot directly from fastboot still works. But the problem with that would be that Magisk would not backup the original boot.img. So you won't be able to restore image from Magisk if at a later stage you decide to unroot. Well, why would you need to unroot? Because if you are on OOS and there is an update, it will fail to proceed because of tampered (patched) boot.img, hence before proceeding with OTA, you would need to restore original boot.img. That is where temporarily booting and then rooting method pays off. By the way, you can restore original boot.img by flashing if you didn't go with temporary boot method.