Displaying selection of sub-categories within a Card

I am working on application that analyzes live video and can tell, with the help of AI, when an object of a certain type (e.g human, vehicle) is detected in the video.

The application will have a wizard to get users started. And looks like this:

However, the project manager now wants the application to allow sub-categories to the Vehicles category, such as Cars, Trucks, Bikes, Busses, etc. How can I best convey to users that the Vehicles category has sub-categories and that they can select one or more of said sub-categories?
Just adding more cards to the list feels wrong since hierarchically these are "below" Vehicles. I also want to keep all of this on "Step 2".

I have been experimenting for 2 days and have tried flipping, expanding or otherwise transforming the card to reveal these options, but nothing "clicked". I am mentally drained of ideas.

Overall UI/UX feedback is also welcome.