Please add a folder called End, all schematics of which will spawn only in the end dimension on end stone.

Do you mean in default schematic pack?
It already exists. Just allow The End dimension in config file

Yes, but then everything spawns there and looks weird. I would like stuff that's End-flavoured to spawn in the End, which is quite a dull place really and could use some more content.

You can create "end" folder with your End-flavoured schematics (in subfolder). It will spawn in the end. code