I spent a few good hours trying to move / off of my SD onto my HDD with no luck. I've been able to do it using Armbian and DietPi, but Arch seems to be handling the boot and mounting differently. Even differently from regular Arch, so no luck with the wiki.
/boot/ does not contain anything like cmdline.txt, however I found /proc/cmdline, which I am unable to edit. So I have tried copying it to /boot/ and making my changes there. After which rebooting and running df -h shows / is still on my SD.