The thing is Egypt has tried breaking free before, didn't work because Rhome can pretty easily fuck them up because of how many troops they have stationed in the despotate and a shit ton of bases in the their direct holding of alexandria. Ethiopia has no reason whatsoever to mess with Rhome they rely so heavly on them for trade that Gondor would basically be shooting itself in the footShit.
But I hope they will at least integrate Egypt a little further, otherwise It will be kinda dumb. This would realistically be the first priority of any sane emperor who gets the throne, more conquests are meaningless before solving this issue.
The way things are now both the Egyptians and the Ethiopians can unilaterally fuck up the Indian trade route if they go rogue, not a very comfortable situation.