Welcome to the Decker community!

There isn't currently any mechanism for visually grouping sets of cards within a deck while editing.

If you're comfortable with programming, you might be able to achieve the project structure you describe by writing pieces of a large game as separate decks and then using a Lilt script to glue the pieces together; Lilt has most of the same scripting interfaces as Decker itself and can also copy and paste cards, widgets, and other resources between decks. I can provide more detailed examples if needed.

From a scripting perspective it's fairly easy to write queries that find cards based on a naming convention. For example, in Valentine-er all the cards containing border designs use a "back_" prefix in their names, and the script which populates the list of options does essentially:

select where key like "back_*" from deck.cards

If you happen to remember the names of cards within a project, you can also use the Listener to quickly navigate to them via the keyboard like so:

Does any of that help?