Update: I have technically resolved the problem(?). Reinstalling xf86-video-amdgpu, mesa, and amdvlk fixed something, so things are running normally when I use the regular kernel, but zen still has the same performance degradation as of 6.4.9 (another log pastebin here ).
I’m still left scratching my head, but maybe this’ll be relevant to someone somewhere somewhen. /shrug