I think I’m going to use Data->Group Items more. It’s a feature I don’t think I’ve ever used in the past.
If you want to add a sibling to an existing document, you can select the existing document and click Data->Group Items. With only one thing selected, Group Items will make a new group named the same as the selected item, moving the item into the new group.
A very minor convenience, but nice. It reduces the “friction” of creating a tree of notes.
And, of course, Group Items works on tags, too, either individually or in batches.