I played around with the system save slot system, and I'm kind of torn between the two.
The only issue with the save slot is that it would load all objects, even those that have the No Save behavior, since its just about the state of that object. Of course we're talking about the editor rather than run-time, so that's different. If the objects in the json didn't exist, they couldn't be placed in the editor, and I'm guessing that if they existed in the project, and didn't exist in the current layout, they would not get created by loading by json. My opinion on that anyways.
Then as a single object.. you would have to load each one separately.
To be honest I kinda think if you wanted to go down the path of pre-loading, you would most likely do more
than one.
Edit:
Also I'm not sure how instances should be handled in the save state load. I mean by my definition you would have to have the exact amount of instances if the save had multiples, which is kinda ehh.