I’d suggest to rather have something like this:
Better an additional “Master Fader”, so you can easily correct the matching between specific zones.
Trouble is, every device has its own “step width” (i.e. how many dB does a step up or down mean). So if the levels are matching for all zones at - say “50” it doesn’t necessarily mean it still matches at “30” or “70”. Icing on the cake: have the step width adjustable for each zone (and share the data for known volume controllers / implement them for “roon ready” devices). I know of some devices that even have split ranges (say 1.5 dB from 0 to 40, 0.5 dB from 41 to 100). Such ranges should be reflected as well.
If one zone runs into a limit (< 0 or > convenience level) it should remain there but remember the relation to the others. Meaning if the master returns to a level that allows it, pick up the previously “stuck” zone volume level and maintain the relation until a single zone fader is changed.