I don't see the need to have the king kicked out of his estates when he loses the crown, as long as he loses his tax immunity. And indeed, you'd need it for it to be shrinkable by the ruler himself, with the lord able to enlarge it if he wants.
Following the idea, a ruler could be appointed as lord of a region, making it an Imperial Region (yes, bringing the concept back) whose taxes go fully into Realm Share, not incurring in the penalties as Imperial Regions ruled by non-king lords.
