I actually can see Charles deciding to open Balmoral year round and only use it as a residence when he needs to such as when the Prime Minister (and family) make a summer visit to the monarch. Charles is a man that feels totally and completely at home in Scotland and he and Camilla use Birkhall as their "getaway" more than anywhere else.
Charles was/is passionate about what he's done with Highgrove but I don't think he's ever thought of it as his own. It was a Duchy investment that Charles worked with and did things with that were a test station for more sustainable use of land. It was more a project than a private property. Same thing with Dumfries House. Charles' aim was to work to make things better, work to preserve things and leave it for future generations.
I think he'll be the same way as a king. He may decide that making Buckingham Palace a more "work" place for the British monarchy along with having it open to the people the monarchy serves gives the air of sustainability to the monarchy of the 21st century. I don't think Charles would be the type to "demand" a home suitable for a king but rather be a king that does what a king should do in service to Crown and country. If he's happy at Birkhall, why would he want to give it up. Open Balmoral to Scotland and only use it himself when being the monarch requires it of him.
As he's totally rewritten the job description of being a Prince of Wales, I will not be surprised if in some ways, he does the same thing as king and monarch.