There is a (limited) solution to this in the HDMI 2.1 spec that’s starting to become available in TVs now: https://www.hdmi.org/spec21sub/quickmediaswitching

It doesn’t solve long initial syncs but it does solve the desync when changing framerate, e.g. when you switch from a 60Hz UI to a 24Hz movie.

As for desyncs between SDR and HDR, that is presumably already a solved problem, I have an LG C1 from a couple of years ago and that can switch between range modes without desyncing.