The frameRendered signal should carry the timestamp from when the Surface got rendered. For e.g. the DRM backend this is on the page flip signal (which carries a timestamp).
Thus the infrastructure needs to be reworked so that Toplevels get marked whether they need a frame rendered callback and for which output they need it.
From my understanding the frame callback is meant to be send as soon as a new commit would not overwrite the previous one in the same frame anymore. This would be in KWin when the scene is fully processed and send to the backend and not when later on the page flip is done by the hardware.
The presentation time protocol in T4455 needs this page flip time though.