I resolved the issue by adding the following line to /usr/share/X11/xorg.conf.d/10-amdgpu.conf-
Option "SWCursor" "True"
I’ve confirmed this does indeed fix Xorg’s extreme CPU usage when moving the mouse around the desktop in Kernel 6.12+ for me. It has the unfortunate side-effect of causing the cursor to flicker wildly in some applications like Steam and occasionally in Firefox, so it’s not quite an optimal fix for me in terms of usability, but it does solve the original issue of single-core CPU spikes. For that reason, I’ll mark this as the solution.