It is possible, but can be a little complex depending on what you’re trying to do. The way we most often approach this is by overriding the template for the default category page in a theme (category boxes with subcategories in your case)… and then setting that as the homepage (controlled by top_menu in Discourse’s admin settings). That template is discourse/categories-boxes.hbs at master · discourse/discourse · GitHub
Template overrides are covered in the advanced section of Developer’s guide to Discourse Themes