Yeahhhhh…. it was the LIBVA_DRIVER_NAME=radeonsi line still saying nvidia in /etc/environment. Which is actually mentioned in the Hardware video acceleration section on the ArchWiki, but I completely overlooked it after everything else seemed to go in circles. And while it feels weird to mark my own reply as the solution, if someone ever changes their nVidia card for an AMD card… this might be your issue.