Hi again! Sorry for the late response, it's been a busy week.
This is just a short reply for now, which may not fit everything you're doing.
I think the group idea you're thinking about is a Contraption.
Basically it's a custom widget made of the other basic types to become something new. Contraptions are things that are meant to be reusable, whether it's something that many kinds of projects like the Calendar example you mentioned or something that can be used many times within one. Like using a contraption to keep track of what items you have. :) So you had the right idea.
Internet-Janitor has just recently posted a contraption version of an Inventory Bar that does some of the things you might want it to do, but not everything.
For example, you can place an InventoryBar on every card and they'll all update each other. And it tracks whether or not something is stored in the inventory so you can write If statements that check it in your code.
But it doesn't use draggable canvases like your project is using doing now, so it would change your gameplay a little bit if you used it as it is.
Is this the kind of thing you were thinking about? (Even if it's not exactly this one.)
On the other hand, there are definitely ways to do this without contraptions. Like having a script that syncs up the locations of your canvases on different cards when you tell it to. But I'd have to come back to write an example.