More info.
journalctl | grep nvidia
gives:
kernel: [drm:nv_drm_atomic_commit [nvidia_drm]] *ERROR* [nvidia-drm] [GPU ID 0x00000100] Flip event timeout on head 0
kwin_wayland[976]: Pageflip timed out! This is a bug in the nvidia-drm kernel driver
kwin_wayland[976]: Please report this at https://forums.developer.nvidia.com/c/gpu-graphics/linux
and
kernel: nvidia-modeset: ERROR: GPU:0: Idling display engine timed out: 0x0000927c:3:0:1264
And some searching, led to this thread on the CachyOS forums:
https://discuss.cachyos.org/t/hard-freezes-under-kde-pageflip-timed-out/28325/2
Suggesting it may be a kwin / nvidia problem, which might match up with my thought it was the latest KDE update that has caused this.