Hi nathan,
You have several options to create the desired module:
In case your categories number is small, you can manually create a button for every category.
For each button, create a hidden repeater, which holds all the topics. onclick/hover, show the repeater.
If you still want to keep your categories dynamic, you can create another table, and use multi-references (CMS: Working with Multiple-Item Reference Fields | Help Center | Wix.com).
Now, you can have two repeaters, one for each collection.
The repeater of the topics will be hidden. again, onclick/hover, you will display the second repeater with the data references from the selected category.
Here is a reference for using repeaters: https://www.wix.com/code/home/repeaters.
I hope you’ll find this useful.
Idan.